Agricultural Engineering · Land & Water Resources Engineering · also Watershed Management, Wasteland Development
Planning a whole watershed: delineating it, classifying its land, budgeting its water and choosing harvesting structures with the people who live there.
In the exams: GATE AG Section 4 Watershed Management; ICAR AIEEA-PG Unit VII; ICAR AICE subject 40 Unit 6; TNPSC Unit III; RPSC AAE Unit 2
A watershed is the area of land that drains to a single outlet: a point on a stream, a lake, a pond or a well-defined gully mouth. Everything that happens on that land, from ploughing up and down a slope to grazing a ridge bare to digging a farm pond, changes how much water leaves at the outlet, when it leaves and how much soil it carries. Watershed planning and management is the discipline that uses this unit to plan land use, soil and water conservation, water harvesting and livelihoods together, instead of one field or one structure at a time.
The subject sits between engineering and people. The engineering half asks you to delineate a watershed from a map or a digital elevation model, describe its shape and drainage with morphometric parameters, classify its land by capability and by suitability for irrigation, draw up a water budget, choose and site harvesting structures, and judge whether the benefits justify the cost. The social half asks you to do all of this with the farmers, herders, landless families and women who live there, because structures that people did not choose are rarely maintained. Most failed watershed projects failed on the second half, not the first.
Indian papers (GATE AG, ICAR AIEEA-PG and AICE-JRF/SRF, ASRB NET, state PSCs) ask short numericals on morphometry, prioritisation, harvesting potential and benefit-cost ratio, and recall questions on land classes and national programmes; licensing exams elsewhere ask the same ideas as planning cases. Runoff estimation belongs to Watershed Hydrology and the design of bunds, terraces and ponds to Soil and Water Conservation Engineering; this manual uses their results and concentrates on planning.
A watershed (called a catchment in British and Australian usage and a drainage basin for large systems) is the land surface that contributes surface runoff to a defined outlet. Its boundary is the water divide, or ridge line: rain falling just inside the divide flows towards the outlet, rain falling just outside flows elsewhere.
Watersheds are nested. A river basin contains catchments, which contain sub-catchments, then watersheds, sub-watersheds, milli-watersheds and micro-watersheds. The size ranges attached to these names differ between agencies and countries, so always state the size of the unit you mean. In India the Soil and Land Use Survey of India (formerly the All India Soil and Land Use Survey, AIS&LUS) prepared a Watershed Atlas that codes every unit in this hierarchy, and national programmes have treated the micro-watershed, commonly of the order of a few hundred hectares, as the planning unit, with projects made up of clusters of micro-watersheds.
The watershed is used as a planning unit because water and sediment cross property boundaries, so a problem at the outlet often starts on the upper slopes; because the outlet is a natural place to measure runoff and sediment; and because work can be ordered from ridge to valley, so structures lower down are not overwhelmed by uncontrolled runoff from above. The limitation is that administrative units (villages, districts) and groundwater aquifers rarely coincide with surface watersheds, so plans must still work through village institutions and consider aquifers that extend beyond the divide.
Manual delineation starts with a topographic map with contour lines, such as a 1:50,000 survey sheet, and a marked outlet. The procedure is: mark the outlet; trace the stream network upstream; locate hilltops and saddles; then draw the divide from the outlet, crossing each contour line at right angles, following the highest ground between the stream network of interest and neighbouring streams, and returning to the outlet on the other side. Runoff moves perpendicular to contours, so a divide that crosses contours obliquely is wrong. The divide never crosses a stream except at the outlet.
In flat land, such as alluvial plains or deltas, contour maps cannot define the divide well and field checks, drainage channels and embankments matter more than topography. In karst country, underground drainage can move water across the surface divide, so the topographic watershed and the groundwater catchment differ.
Once drawn, the watershed area is measured with a planimeter, by counting grid squares, or digitally. The basin length (the distance from the outlet to the farthest point on the divide, measured roughly parallel to the main stream), the perimeter and the stream lengths by order are then measured for morphometric analysis.
Most delineation is now done in a GIS using a digital elevation model (DEM), a grid of elevations; free global DEMs of about 30 m resolution serve for planning, and LiDAR or drone DEMs for micro-watersheds. The standard steps are: (1) Fill sinks: isolated low cells, many of them errors, are raised so that water can flow out. (2) Compute flow direction: in the common D8 method each cell drains to the one of its eight neighbours with the steepest downhill slope. (3) Compute flow accumulation: the number of upstream cells that drain through each cell. (4) Define streams by choosing a threshold of flow accumulation; a small threshold gives a dense network, a large one a sparse network, so drainage density from a DEM depends on the threshold chosen. (5) Snap the outlet (pour point) to the cell of highest accumulation nearby. (6) Delineate the watershed as all cells that drain to that pour point. (7) Order the streams, usually by the Strahler method.
A DEM-derived boundary must be checked. Flat areas, roads, canals and embankments misdirect flow, and a pour point one cell off the stream gives a watershed of a few cells.
Stream order is the basis of morphometry. In the Strahler system, fingertip channels with no tributaries are first order; two streams of order u join to form order u+1; a stream of lower order joining one of higher order does not change the higher order. The order of the main stream at the outlet is the order of the watershed. Exam questions assume Strahler ordering unless they say otherwise.
Characterisation is the systematic description of a watershed before anything is planned. It has a physical part and a human part. The physical part covers location and area, relief and slope, geology and aquifers, soils (depth, texture, erodibility), climate and rainfall, land use and land cover, the drainage network, existing structures and the condition of each (erosion status, gullies, silted tanks, dry wells). The human part covers population, landholding and tenure, cropping and livestock systems, sources of income, migration, access to markets and credit, and existing institutions such as user groups and village councils.
The output is a set of maps and a baseline report; without the baseline, no later evaluation can show what the project changed.
Morphometry is the quantitative part of physical characterisation. It describes the shape, drainage and relief of the watershed with numbers that can be compared between watersheds. The numbers come from Horton, extended by Strahler, Schumm and Miller. They are grouped as linear, areal (shape) and relief parameters.
Stream order (u) is described in the previous chapter. Stream number (Nu) is the number of stream segments of each order. Horton's law of stream numbers states that Nu falls roughly geometrically as order increases.
Bifurcation ratio: Rb = Nu / N(u+1), the number of streams of one order divided by the number of the next higher order. A mean bifurcation ratio for the watershed is often reported, either as the arithmetic mean of the ratios between successive orders or as a mean weighted by the number of streams involved; say which method you used. Values of about 3 to 5 are typical of watersheds where geology has not strongly distorted the drainage pattern. Unusually high values suggest strong structural (geological) control, such as elongated valleys following faults.
Stream length (Lu) is the total length of streams of each order; the mean stream length of order u is Lu / Nu. The stream length ratio RL is the ratio of mean stream length of one order to that of the next lower order.
Length of overland flow: Lg = 1 / (2 Dd), where Dd is drainage density. It is the average distance rain travels over the land before reaching a channel. A short Lg (high drainage density) means water reaches channels quickly.
Drainage density: Dd = ΣL / A, the total length of all streams divided by watershed area, in km per km². High drainage density indicates impermeable subsoil, sparse vegetation, high relief or erodible material and gives a quick, peaky runoff response; low drainage density indicates permeable material and dense cover. Because the value depends on map scale and on the stream threshold used with a DEM, only compare watersheds analysed the same way.
Stream frequency: Fs = ΣN / A, the total number of stream segments per km². Texture ratio is the number of streams (in Horton's usage, first-order streams; some authors use all streams) divided by perimeter; state the definition used.
Form factor (Horton): Rf = A / Lb², where Lb is basin length. A perfectly circular basin has a form factor of about 0.79; smaller values mean more elongated basins. Elongated basins produce flatter, longer peak flows than circular basins of the same area because runoff from different parts reaches the outlet at different times.
Circularity ratio (Miller): Rc = 4πA / P², where P is perimeter. It equals 1 for a circle and falls as the shape becomes elongated or the divide more irregular.
Elongation ratio (Schumm): Re = (2 / Lb) × √(A / π) = 1.128 √A / Lb. It is the diameter of a circle with the same area as the basin divided by basin length. Values near 1 indicate low relief and circular shape; values of roughly 0.6 to 0.8 are associated with high relief and steep slopes.
Compactness coefficient (Gravelius): Cc = P / (2√(πA)) = 0.2821 P / √A, the perimeter divided by the circumference of a circle of equal area. It equals 1 for a circle and is larger for elongated or irregular basins. Note that Cc and Rc are related: Rc = 1 / Cc².
Basin relief (H) is the difference in elevation between the highest point on the divide and the outlet. Relief ratio (Schumm): Rh = H / Lb, dimensionless when H and Lb are in the same unit. It indicates overall steepness and is correlated with sediment loss. Relative relief divides H by perimeter instead of basin length.
Ruggedness number: Rn = H × Dd, dimensionless when H is in km and Dd in km per km². High values occur where slopes are both steep and long and the drainage dense, which is where erosion hazard is high.
Hypsometric analysis describes how area is distributed with elevation. The hypsometric curve plots relative area above each elevation against relative height; the area under it is the hypsometric integral (HI). A convenient approximation is the elevation-relief ratio: E = (mean elevation − minimum elevation) / (maximum elevation − minimum elevation). Strahler interpreted high values (above about 0.6) as a youthful, inequilibrium stage with much material still to be eroded, values between about 0.35 and 0.6 as a mature, equilibrium stage, and low values as an old (monadnock) stage. The interpretation is qualitative; lithology and tectonics also change the curve.
Morphometric numbers are indicators, not predictions; measured erosion and the priorities of residents decide the final plan.
A district or a programme cannot treat every watershed at once, so it ranks them. Prioritisation methods fall into three families.
Morphometric (compound parameter) prioritisation ranks sub-watersheds on several parameters and averages the ranks. Parameters thought to rise with erosion hazard (bifurcation ratio, drainage density, stream frequency, texture ratio, length of overland flow in some studies) are ranked so that the highest value gets rank 1. Shape parameters (form factor, circularity ratio, elongation ratio) are ranked the other way, lowest value rank 1, on the argument that lower values indicate elongated basins with greater erosion hazard; authors differ on this and on how to treat the compactness coefficient, so state your convention. The compound parameter is the mean of the ranks, and the sub-watershed with the lowest compound parameter gets the highest priority.
The sediment yield index (SYI), developed by the AIS&LUS in India, uses soil and land survey rather than stream geometry. The watershed is mapped into erosion intensity mapping units, each with an area (Ai), a weightage value (Wi) that reflects its relative erosion intensity, and a delivery ratio (Di) that reflects how much of the eroded material reaches the reservoir or outlet, depending on distance and channel conditions. Then SYI = [Σ (Ai × Wi × Di) / Aw] × 100, where Aw is the total area of the sub-watershed. The index is relative: it ranks sub-watersheds within one catchment, and sub-watersheds are placed in priority classes (very high, high, medium, low, very low) by bands of the index published in the survey's guidelines.
Other approaches estimate soil loss per sub-watershed with the Universal Soil Loss Equation, or weight layers (slope, soil, land use, groundwater decline, poverty) by multi-criteria methods such as the analytic hierarchy process. National programmes add socio-economic criteria such as rainfed area and drinking water scarcity, because the aim is livelihoods, not only less sediment.
Land capability classification (LCC) groups land by its capacity to produce common crops and pasture over a long period without deterioration. The system most widely taught was published by Klingebiel and Montgomery for the USDA Soil Conservation Service in 1961 (Agriculture Handbook 210) and has been adapted by many countries, including India.
LCC is interpretive: it reads soil survey data (depth, texture, slope, erosion, wetness, stoniness, salinity, climate) as limitations to use, assumes a moderately high level of management, and ignores location, markets and ownership.
There are three levels. The capability class (I to VIII) gives the degree of limitation. The capability subclass gives the kind of limitation: e for erosion hazard, w for excess water (wetness, drainage, overflow), s for limitations in the soil root zone (shallow, stony, low water-holding, salinity or sodicity), and c for climatic limitation (temperature or rainfall). Class I has no subclasses because it has no significant limitations. The capability unit groups soils within a subclass needing the same management.
Classes I to IV are suited to cultivation. Class I has few limitations. Class II has some limitations that reduce the choice of crops or need moderate conservation practices. Class III has severe limitations that reduce the choice of crops, need special conservation practices, or both. Class IV has very severe limitations and needs very careful management; it is suited to occasional cultivation or a narrow range of crops.
Classes V to VIII are not suited to cultivation. Class V has little or no erosion hazard but other limitations, such as frequent flooding, wetness or stones, that cannot practically be removed; it is used for pasture, range or woodland. Class VI has severe limitations that make it generally unsuited to cultivation and limit it to pasture, range, woodland or wildlife. Class VII has very severe limitations that restrict it to grazing, woodland or wildlife under careful management. Class VIII has limitations that preclude commercial plant production and restricts it to recreation, wildlife, water supply or aesthetic purposes.
On maps each class has a conventional colour, widely taught as light green for class I, yellow for II, red for III, blue for IV, dark green for V, orange for VI, brown for VII and purple for VIII. Maps show the class and subclass, for example IIIe or IVes, with the dominant limitation first.
The value of LCC in watershed planning is that it ties treatment to land rather than to ownership. A field classed IIIe needs contour cultivation, conservation bunds or strip cropping if it is to be cropped safely; a slope classed VIe should not be cropped at all, and if it is, the plan must either convert it to perennial vegetation or provide far heavier treatment. Much degradation is land used beyond its capability, such as steep class VI and VII land cultivated because the family has no other land.
LCC has limits. It was designed for American farming at moderate to high management, so its thresholds (for slope, depth and erosion) are adjusted in each country's soil survey manual; do not assume the slope limits of one national manual apply elsewhere. It says nothing about which crop pays best. And it treats land as fixed, whereas terracing, drainage or reclamation can change the effective limitation, which is why the classification of reclaimable land is sometimes revised after treatment.
The FAO Framework for Land Evaluation (1976) takes a different approach: instead of asking what the land can support in general, it asks how suitable it is for a defined land utilisation type, such as rainfed maize under smallholder management or irrigated rice. Suitability is given as orders: S (suitable) and N (not suitable). Within them the classes are S1 (highly suitable), S2 (moderately suitable), S3 (marginally suitable), N1 (currently not suitable, but the limitation could be overcome in the future) and N2 (permanently not suitable). Subclasses show the kind of limitation, as in LCC.
Because the same land may be S3 for maize but S1 for a fodder tree, watershed plans often use LCC to find where cultivation is safe and suitability analysis to choose what to grow there.
The treatment plan is the core document of a watershed project. It overlays the land capability map, the slope map, the land use map and the drainage network, and assigns a treatment to every parcel and every drainage line. It is organised in three layers, worked from ridge to valley: area treatment on non-arable land (the upper slopes, usually common or forest land), area treatment on arable land (the middle slopes, usually private farms) and drainage line treatment (the gullies and streams that collect runoff from both).
The basic rule is to match use and treatment to capability. Land cropped within its capability gets agronomic and light mechanical measures; land cropped beyond it is shifted to a less demanding use (perennial fodder, fruit trees, agroforestry) or given heavier treatment such as bench terraces. Drainage line structures are sized for the runoff that upper treatments still let through.
A good plan also records, for each parcel, the owner or user, their agreement, their contribution and who will maintain the work.
On arable land (roughly classes I to IV) the aim is to keep rain where it falls, reduce soil loss to a tolerable level and raise yields.
Agronomic measures come first because they are cheap and farmers already understand them: contour cultivation (ploughing and sowing across the slope), strip cropping (alternating strips of erosion-permitting and erosion-resisting crops across the slope), cover crops, mulching, conservation tillage, intercropping, and balanced nutrients that give denser canopies.
In-situ moisture conservation measures shape the land surface to hold water: ridges and furrows, tied ridges, compartmental bunding, broad bed and furrow on heavy clay soils, dead furrows, and conservation furrows.
Mechanical measures on arable land include contour bunds on low-rainfall permeable soils, graded bunds that lead excess water safely to a waterway on higher-rainfall or less permeable soils, and bench terraces on steeper land where cultivation is unavoidable. Grassed waterways carry the surplus to the drainage line. Field boundary bunds with vegetative barriers of grasses or hedgerows are a lower-cost option that farmers often prefer to earthen bunds, because they take less land out of cultivation.
The choice among these depends on slope, soil depth and permeability, rainfall and the farmers' willingness to lose land to structures; design details (bund spacing, cross-section, terrace dimensions, waterway sizing) are covered in Soil and Water Conservation Engineering.
On non-arable land (roughly classes V to VIII, and degraded common land in general) the aim is to re-establish perennial vegetation and to trap runoff and sediment near where they start.
Measures include continuous or staggered contour trenches, which intercept runoff and give seedlings water; micro-catchments such as half-moon (semi-circular) bunds, V-ditches and eyebrow terraces around individual trees; pits for tree planting; stone walls and vegetative contour lines; and closure of the area to grazing and cutting for a period, enforced by agreement of the community (social fencing) rather than by physical fencing alone. Revegetation uses grasses, fodder shrubs and multipurpose trees chosen with the users, so that the land yields fodder, fuel or fruit and people have a reason to protect it. Silvipasture (trees with pasture) and horti-pasture are common end uses.
Drainage line treatment works from the top of the gully downwards. Gully heads are stabilised to stop them cutting back. Brushwood, loose boulder and gabion checks slow flow and trap sediment in small gullies. Earthen or masonry check dams and nala bunds on larger streams store water and raise groundwater levels. Percolation tanks are placed where the underlying strata are permeable, and farm ponds on private land where a farmer wants stored water. Each structure is designed for the runoff expected after upstream treatment, and a spillway or safe outlet is always provided; an overtopped earthen structure fails and releases stored sediment.
Because benefits from non-arable land are shared and delayed, this treatment depends heavily on participatory institutions.
Land that is good for rainfed crops is not necessarily good for irrigation. Irrigation adds water that must drain away, can raise the water table, can bring salts to the surface and costs money to deliver. Land irrigability classification asks whether land can be irrigated in the long term without deterioration and whether it will repay the investment. In a watershed plan it is used when deciding where water harvested in ponds, tanks or recharged wells should be used, and in larger schemes when deciding which land a canal command should include.
The factors are soil (depth, texture, water capacity, infiltration, permeability, salinity, sodicity, stoniness), topography (slope, micro-relief, levelling needed), drainage (internal drainage, water table depth, waterlogging risk, drainage outlet) and irrigation water quality (salinity, sodium adsorption ratio, boron).
The United States Bureau of Reclamation (USBR) developed the classification most often taught for irrigation projects. It is economic as well as physical: classes are defined by their ability to repay the costs of irrigation.
Class 1 (arable) is land highly suitable for irrigated farming, able to sustain high yields of a wide range of crops at reasonable cost. Class 2 (arable) has moderate suitability and a lower repayment capacity, because of limitations in soil, topography or drainage that can be corrected at moderate cost or that narrow the range of crops. Class 3 (arable) has restricted suitability, with more severe limitations, but is still suitable for irrigation development with adequate management. Class 4 (limited arable or special use) is suited only to a limited range of uses, such as particular crops or pasture, or needs special practices; it may have an excessive deficiency that is correctable only at high cost. Class 5 (non-arable) is land that is temporarily classed non-arable pending further investigation, or requires special study, for example land whose drainage or reclamation feasibility is not yet known. Class 6 (non-arable) is land that does not meet the minimum requirements of the other classes and is not suitable for irrigated farming.
Subclasses show the deficiency: in common adaptations, s for soil, t for topography and d for drainage.
The Indian soil survey manual first groups soils into soil irrigability classes, lettered from A for soils with no limitation for sustained irrigation through progressively more severe limitations to soils not suitable for irrigation. It then combines soil irrigability with water availability, water quality and drainage to give land irrigability classes numbered 1 to 6, following the USBR pattern. The FAO land evaluation approach can also be applied with irrigated agriculture as the land utilisation type, giving S1 to N2 suitability classes for surface, sprinkler or drip irrigation separately, which is useful because land unsuitable for surface irrigation (steep, shallow, very permeable) may be suitable for drip.
In practice: irrigate deeper, better-drained valley soils rather than shallow upland soils, check the water table in low-lying land, test new well and pond water for salinity and sodium, and prefer pressurised methods on sloping or light soils.
A water budget is an accounting of all water entering, leaving and stored in a defined area over a defined period. For a watershed over a year the general equation is: inflow = outflow + change in storage. Written out, P + Qin + Gin = ET + Qout + Gout + ΔS, where P is precipitation, Qin and Qout are surface inflow and outflow, Gin and Gout are groundwater inflow and outflow, ET is evapotranspiration and ΔS is the change in storage (soil moisture, groundwater, surface reservoirs). For a watershed bounded by a topographic divide, Qin is usually zero; if Gin and Gout are small or equal, the balance simplifies to P = ET + Q + ΔS, and deep percolation to aquifers is part of ΔS or a separate term R.
All terms are expressed in the same unit, usually millimetres of depth over the watershed or cubic metres. To convert: volume (m³) = depth (mm) ÷ 1000 × area (m²), and 1 mm over 1 hectare is 10 m³. Most arithmetic errors in water budgeting are unit errors between mm, m, ha and m³.
Estimating each term is covered in Watershed Hydrology; planning uses the estimates to decide how much runoff can be harvested, how much recharge can be pumped and whether planned crops and livestock can be supplied.
A planning water budget sets supply against demand. Supply includes harvestable surface runoff, groundwater recharge that can be safely pumped, and existing tank and pond storage. Demand includes drinking and domestic water for people, water for livestock, crop water requirements under the planned cropping pattern, and environmental flows downstream.
Domestic demand is estimated from population and a per-capita norm. Norms differ by country and programme: in India, rural drinking water programmes have used 40 litres per capita per day in earlier guidelines and 55 litres per capita per day under the current tap-water mission. Livestock demand uses norms per animal that vary with species, breed, climate and feed; take them from local guidelines. Crop demand is the net irrigation requirement (crop water requirement minus effective rainfall) divided by the application efficiency of the irrigation method.
Draw up the budget for the year and for the critical dry season separately, because an annual surplus can hide a summer deficit. The result shows whether the priority is to harvest more water, to reduce demand (other crops, drip irrigation) or both.
Harvesting more runoff upstream reduces flow downstream. In a closed basin, where downstream users already use most of the flow, new upstream structures reallocate water rather than create it.
A groundwater balance for the watershed compares recharge (from rainfall, from ponds and tanks, from return flow of irrigation) with draft (pumping) and natural discharge. The ratio of annual draft to net annual available recharge is the stage of groundwater development used by groundwater agencies to categorise areas; in India assessment units are grouped as safe, semi-critical, critical and over-exploited, by stage of development and by the long-term trend of water levels. Where development is already high, a watershed plan should concentrate on recharge and demand management rather than on new wells.
In community water budgeting, villagers measure rainfall and well levels and decide together how much area of each crop the water can support; the engineer supplies the method, the users make the decision.
The budget also sets a ceiling for evaluation: if most rainfall returns to the air as evapotranspiration, the gain from harvesting is a share of the remaining runoff and recharge, and claims of large increases in irrigated area should be checked against it.
Water harvesting is the collection of runoff for productive use. Critchley and Siegert's FAO manual groups it by the distance between catchment and use. In micro-catchment harvesting, runoff is collected from a small area next to the plant, within the field: contour ridges, semi-circular bunds, negarim micro-catchments (diamond-shaped basins around a tree), zai or planting pits, and in-situ systems such as tied ridges. In external catchment or macro-catchment harvesting, runoff from a larger area, often a hillside or a road, is led to cropped land or stored: trapezoidal bunds, contour stone bunds, water-spreading and flood-water harvesting from ephemeral streams. In storage systems water is held in a structure for later use: farm ponds, tanks, check dams, cisterns.
In the Thar desert of Rajasthan, khadin are earthen embankments across valley floors that pond runoff and allow a crop on the moist bed after the water drains; nadi are village ponds; tanka are covered underground cisterns fed by a prepared catchment. South Indian tank (eri) systems store monsoon runoff in chains of tanks. Johad in Rajasthan and Haryana are small earthen check dams. Outside South Asia, the jessour of southern Tunisia are dry-stone check dams behind which terraced fields form in wadis; zai pits and stone lines have been widely used in the Sahel, particularly in Burkina Faso and Niger.
A check dam is a small barrier across a drainage line. Its purposes are to reduce the velocity of flow and so reduce channel erosion, to trap sediment, to store water for a short period for protective irrigation or livestock, and to increase recharge. Small checks (brushwood, loose boulder, gabion) are built in first- and second-order gullies; larger earthen or masonry check dams and cement-plug structures are built on second- to fourth-order streams. A nala bund is an earthen embankment across a nala (stream) for the same purposes.
Good check dam sites have a narrow channel with stable, high banks, so a short structure stores a long pool; a gentle bed slope; a firm foundation; enough catchment to fill it; and wells or farmers nearby to use the water. In a series, each pool should reach roughly to the toe of the structure above, and every check dam must pass the design flood safely.
A percolation tank is a storage whose main purpose is to recharge groundwater, not to supply water directly. Siting is decided by the geology below the water spread: fractured or weathered hard rock, sandy or gravelly strata that transmit water to the aquifer, with wells downstream that will benefit. A tank on clay or impervious rock stores water but recharges little. Tanks that fill and empty several times in a season recharge more than tanks holding water into the dry season, when it evaporates, and silt sealing the bed makes desilting part of maintenance.
A farm pond is a small storage on or beside a farm that collects runoff for protective (lifesaving) irrigation, livestock, fish or domestic use. Dugout ponds are excavated in flat or gently sloping land; embankment ponds are formed by a bund across a depression or drainage line; some combine both.
Planning a farm pond means answering four questions. Will the catchment fill it? Seasonal runoff = runoff coefficient × rainfall × catchment area. Is enough water left after losses? Seepage in light soils and evaporation can remove a large share; lining with compacted clay, soil-cement or plastic membranes reduces seepage. Will the water be used efficiently? One lifesaving irrigation at a critical crop stage often returns far more per cubic metre than full irrigation, and drip or sprinkler stretches it further. Is it safe? The inlet needs a silt trap or grassed filter, a spillway must pass excess flow, and deep ponds need fencing.
The volume of a pond with sloping sides is computed with the prismoidal formula, V = (h / 6)(A1 + 4Am + A2), where A1 and A2 are the top and bottom areas and Am is the area at mid-depth. The simpler average end-area formula, V = h (A1 + A2) / 2, overestimates the volume of a pyramid-like frustum. Structural details (side slopes, freeboard, embankment design, spillway sizing) are covered in Soil and Water Conservation Engineering.
Roof-top rainwater harvesting collects rain falling on roofs for domestic use or for recharging groundwater. A system has a catchment (the roof), conveyance (gutters and downpipes), a first-flush device and filter, storage (a tank or cistern) or a recharge structure, and an overflow. In India the Bureau of Indian Standards has published guidelines for roof-top rainwater harvesting (IS 15797), and many municipalities require harvesting systems in new buildings; Tamil Nadu made roof-top harvesting compulsory across the state in the early 2000s.
Hard, smooth roofs (galvanised iron, tiles, concrete) give cleaner water and higher runoff coefficients than thatch; lead paints and lead flashing should not be used on roofs supplying drinking water.
The first flush after a dry spell washes dust, droppings and leaves off the roof; a first-flush diverter discards it before the tank, the volume depending on how dirty the roof is and how long the dry spell was. Filters range from a mesh screen to layers of sand, gravel and charcoal. Drinking water should still be treated, because roof water can carry bacteria.
The harvesting potential of a roof is: V = A × R × C, where A is the plan (horizontal projected) area of the roof, R the rainfall and C the runoff coefficient. With A in square metres and R in millimetres, A × R gives litres directly, because 1 mm on 1 m² is 1 litre. Runoff coefficients for hard roofs are commonly taken in the range of about 0.75 to 0.9, lower for rough or porous surfaces, and they include losses to wetting, splash, evaporation and gutter overflow in heavy storms. Use locally measured values where they exist.
The plan area is used, not the sloping surface area, because the rain a roof intercepts depends on its horizontal footprint. A roof of 10 m × 12 m has a plan area of 120 m²; in a year with 800 mm of rain and C = 0.8 it can yield 120 × 800 × 0.8 = 76,800 litres, or about 77 m³.
If annual yield is below annual demand, the roof, not the tank, is the limit, and a bigger tank will not help.
Tanks are sized by one of three methods. The simplest, demand-side method, sizes the tank to meet demand through the longest typical dry period: storage = daily demand × number of dry days. It suits climates with one clear dry season and assumes the tank is full at the start of the dry season, which must be checked against roof yield. The supply-side or mass-curve method plots cumulative roof yield and cumulative demand month by month; the largest gap between the two lines, measured when demand exceeds supply, gives the storage needed. The most thorough method simulates the tank day by day or month by month with a long rainfall record (a behavioural model), giving the reliability, that is the percentage of days on which demand is met, for each tank size. Because tanks are the most expensive part of the system, the trade-off between size and reliability is a real design decision.
Where water is not needed for direct use, as in many cities with piped supply, roof water is led to recharge structures: recharge pits filled with graded boulders, gravel and sand; recharge trenches; or existing or purpose-drilled recharge wells or borewells, always through a filter so that silt does not clog the aquifer. Recharge structures should not be placed where the water table is very shallow, near septic tanks or soak pits, or where polluted runoff from roads or industries can enter.
Early programmes designed by engineers and built by contractors often failed in the same way: bunds built without farmers' agreement were ploughed out, grazing bans imposed on herders were ignored, and common assets were not maintained after the project left. Pilot projects from the 1980s in India, Kenya, Burkina Faso and elsewhere did better when villagers chose the treatments, shared the cost and enforced their own rules.
Participation ranges from passive (people are told), through consultation and participation for material incentives (labour for wages), to functional participation (groups formed to meet project goals) and self-mobilisation (people act independently). Watershed programmes aim for the upper end.
Benefits are unevenly distributed: valley farmers gain from recharge and stored water, while those who give up grazing on the upper slopes, often landless or pastoral families, bear the cost. Plans that ignore this lose the cooperation of those who control the upper slopes.
Participatory rural appraisal (PRA), developed and popularised by Robert Chambers and others, provides methods through which villagers analyse their own situation. The tools used in watershed planning include:
Transect walks, in which a team of villagers and staff walk across the watershed from ridge to valley and note soils, land use, problems and opportunities in each zone. Social maps, drawn by villagers on the ground or on paper, showing households, castes or groups, and facilities. Resource maps showing land, water sources, forests, grazing land and eroded areas. Seasonal calendars of rainfall, work, food shortage, migration, disease and prices. Wealth or well-being ranking, which identifies the poorest households so they are not left out. Venn (chapati) diagrams of the institutions that matter to villagers and how close or distant they are. Matrix and pairwise ranking to choose among options, for example among tree species or structure types. Timelines and trend lines of history, droughts and changes in wells or forest.
Results are triangulated against measurements and against the views of women and the landless, often absent from village meetings.
Typical institutions in Indian programmes are a village watershed committee that plans and manages works; user groups of those who benefit from a structure or resource; self-help groups, often of women, for savings and livelihoods; and a project implementing agency with a multi-disciplinary watershed development team.
Projects usually move through three phases. The preparatory phase covers entry point activities (small works of immediate value to the whole community, such as repairing a drinking water source, used to build trust), institution building, baseline surveys and the participatory preparation of a detailed project report. The works phase implements the treatment plan and livelihood activities. The consolidation and withdrawal phase completes works, hands over assets and funds, and strengthens institutions so they can manage without the project.
Contributions from beneficiaries, in cash or labour, are required by most programmes as a test of real demand and to create a watershed development fund for later maintenance. Under India's Common Guidelines for watershed development, beneficiaries have contributed a percentage of the cost of works on private land, with a lower rate for small and marginal, scheduled caste and scheduled tribe farmers, and no contribution has been required for works on common land. Check the current guidelines for the rates in force.
At Sukhomajri in Haryana, a small dam was built in the late 1970s to reduce siltation of a downstream lake; the village agreed to stop grazing the catchment, and water rights from the dam were shared among all households, including the landless, who could trade them. At Ralegan Siddhi in Maharashtra, community leadership combined watershed works with rules on grazing, tree cutting and crop choice. Both depended on strong local leadership and rules that the community itself enforced, which are not easily replicated by decree.
Cost-benefit analysis (CBA) compares the value of what a project produces with the value of what it uses, both measured over the life of the project. Gittinger's World Bank text remains the standard reference for agricultural projects.
The correct comparison is with and without the project, not before and after. Many things change over a project's life anyway (prices, rainfall, new varieties); only the difference between the situation with the project and the situation that would have existed without it is a project benefit. In practice the without-project situation is estimated from a comparable untreated watershed, from trends before the project, or from farm models.
Costs include investment in structures, vegetation and institutions; operation and maintenance each year; the value of land taken out of cultivation by bunds and ponds; and the value of grazing or fuelwood given up during closure. Benefits include increased crop yields and area, a second crop from stored or recharged water, fodder, fuel and timber from treated common land, reduced soil loss (valued as nutrients saved or as productivity maintained), reduced siltation of downstream reservoirs, higher and longer-lasting well yields, and reduced flood damage.
Financial analysis uses the farmer's or agency's viewpoint and market prices. Economic analysis takes society's viewpoint, removes transfers such as taxes and subsidies, and uses shadow prices where market prices are distorted, for example unskilled labour below the market wage where unemployment is high.
Money received later is worth less than money received now, so future costs and benefits are discounted. The present value of an amount F received in year t at discount rate r is PV = F / (1 + r)^t. The present value of a constant annual amount A for n years (the uniform series or annuity) is PV = A × [1 − (1 + r)^−n] / r.
Three measures follow. Net present value (NPV) is the present value of benefits minus the present value of costs; a project is acceptable if NPV is positive at the chosen discount rate. Benefit-cost ratio (BCR) is the present value of benefits divided by the present value of costs; a project is acceptable if BCR is greater than 1. Internal rate of return (IRR) is the discount rate at which NPV equals zero; a project is acceptable if IRR exceeds the opportunity cost of capital. IRR is usually found by trial: compute NPV at two rates, one giving a positive and one a negative NPV, and interpolate linearly between them.
The measures can disagree when ranking projects of different sizes; NPV is preferred among mutually exclusive projects. The BCR depends on whether operating costs are subtracted from benefits or added to costs, so always state how it was computed. The payback period, investment divided by annual net benefit, ignores discounting and benefits after payback and is used only as a rough check.
Watershed projects have large early costs and benefits that build up over years, so a high discount rate penalises slow-maturing benefits such as tree growth and reduced siltation. Sensitivity analysis, recomputing NPV or BCR with benefits reduced, costs raised or benefits delayed, shows how robust the result is. A switching value, the change in a variable that brings NPV to zero, is a useful way to present this.
Monitoring tracks inputs, activities and outputs during a project: money spent, works completed, trees surviving. Evaluation periodically assesses outcomes and impacts: did groundwater, cropped area, yields, incomes and soil loss change, and did the project cause it. A baseline survey at the start, a mid-term evaluation and an end-term evaluation are standard; ex-post evaluation some years after closure tests sustainability.
A logical framework (logframe) links goal, purpose, outputs and activities with measurable indicators, means of verification and assumptions for each level. Typical watershed indicators include depth to water in observation wells before and after the monsoon, area irrigated and cropping intensity, crop yields, fodder availability, vegetation cover (increasingly from satellite vegetation indices), sediment deposited in tanks, runoff and soil loss measured at a gauging station at the outlet, household income and migration, and the functioning of watershed institutions.
Attribution is the hard part. Rainfall variation can swamp project effects, so indicators must be compared with a control. Designs include paired watersheds (a treated and a similar untreated watershed monitored together, with a calibration period before treatment), before-after-control-impact comparisons, and statistical methods using matched households. Participatory monitoring, in which user groups record well levels, rainfall and yields themselves, builds local ownership and gives the community the information it needs to manage water after the project ends. Satellite data track land use and vegetation cheaply but still need ground truth.
India has one of the longest records of state-funded watershed programmes. The centrally sponsored scheme of soil conservation in the catchments of river valley projects began in the early 1960s to reduce sedimentation of major reservoirs. The Drought Prone Areas Programme (DPAP, 1973–74) and Desert Development Programme (DDP, 1977–78) moved towards area development of drought-prone and desert regions. The National Watershed Development Project for Rainfed Areas (NWDPRA, launched 1990–91) under the Ministry of Agriculture and the Integrated Wastelands Development Programme (IWDP, from 1989–90) extended the approach.
The report of a committee chaired by C. H. Hanumantha Rao led to the 1994 common guidelines, which made the watershed the unit for DPAP, DDP and IWDP, with village watershed committees, user groups and beneficiary contributions. The Common Guidelines for Watershed Development Projects of 2008 (revised 2011) brought the programmes of the Department of Land Resources together as the Integrated Watershed Management Programme (IWMP), with larger clusters of micro-watersheds, a phased project cycle, dedicated institutions at state and district level, and more emphasis on livelihoods for the landless. In 2015–16 IWMP became the Watershed Development Component of the Pradhan Mantri Krishi Sinchayee Yojana (WDC-PMKSY), and a second phase (WDC-PMKSY 2.0) followed for new projects from 2021–22. Large amounts of water conservation work are also done under the rural employment guarantee scheme (MGNREGS).
Evaluations, including ICRISAT's meta-analyses, report positive average returns with wide variation between projects.
China's Loess Plateau, one of the most erodible landscapes in the world and a major source of the Yellow River's sediment, was the site of the World Bank-supported Loess Plateau Watershed Rehabilitation Project from the mid-1990s. It combined terracing of sloping farmland, check dams (including warping or silt-trapping dams that create new flat fields from trapped sediment), afforestation and grassland restoration with a ban on free grazing, and it was widely cited as showing that large-scale restoration could raise incomes and reduce sediment together. China's national Sloping Land Conversion Programme ("Grain for Green"), launched in 1999, paid farmers to convert steep cropland to forest or grassland.
In the United States, the Soil Conservation Service was created in 1935 after the Dust Bowl, and renamed the Natural Resources Conservation Service (NRCS) in 1994. The Watershed Protection and Flood Prevention Act of 1954 (Public Law 83-566) funds small watershed projects planned with local sponsors, and conservation districts give local farmers a formal role. The Tennessee Valley Authority (1933) is an early example of basin-scale planning of a whole river system.
In Australia, the Landcare movement, established nationally around 1989, organised farmers and community groups to address land degradation, salinity and erosion at catchment scale, and catchment management authorities (with varying names by state) now plan natural resource management by catchment.
In Ethiopia, large-scale soil and water conservation on communal land has been carried out through food-for-work and cash-for-work programmes, including the MERET programme supported by the World Food Programme and later the public works of the Productive Safety Net Programme; the national Sustainable Land Management Programme and community watershed guidelines use a participatory micro-watershed approach. Kenya's National Soil and Water Conservation Programme, supported by Swedish aid from the 1970s, promoted fanya juu terraces (built by throwing soil uphill from a trench) and later adopted a catchment approach in which whole communities planned together. In the Sahel, farmer-led spread of zai pits, stone bunds and farmer-managed natural regeneration of trees has restored degraded land in Burkina Faso and Niger.
In southern Brazil, micro-watershed programmes in the states of Paraná and Santa Catarina, supported by the World Bank from the late 1980s, combined no-tillage, cover crops and rural road drainage at micro-watershed scale and contributed to the wide adoption of no-till farming. Nepal created a Department of Soil Conservation and Watershed Management in 1974, and its programmes work through community user groups, building on the success of community forestry.
The common lessons: technical measures succeed where users have secure rights to the benefits; grazing closures work when users agree and see fodder benefits quickly; the landless and women must be included; common land and upper slopes matter as much as private farms; monitoring must run long enough to see slow benefits; and maintenance after withdrawal, which depends on local institutions and funds, is the real test.
Learn the formulas of each morphometric parameter from their definitions, not by rote: a form factor is area over length squared because it compares the basin with a square of side equal to its length. Practise delineating a watershed by hand on a contour sheet at least once before using a GIS, because it teaches you why the divide crosses contours at right angles and why DEM outputs need checking. Work every harvesting and water budget question in consistent units, writing mm, m, ha and m³ at each step. Memorise the eight land capability classes with one example use each, the USBR classes 1 to 6, and the sequence of Indian watershed programmes, because recall questions on these are common and quick marks.
A watershed engineer five years into practice prepares detailed project reports: delineating micro-watersheds from a DEM, preparing slope, land use and capability maps, running PRA exercises with the watershed development team, and turning all of it into a treatment plan and estimate. Typical decisions are where a check dam will actually recharge wells, whether a farmer's field needs graded bunds or only vegetative barriers, how big a farm pond the catchment can fill, and how to stagger works so that ridge treatment precedes drainage line structures. The practitioner also measures completed works for payment, checks that spillways are adequate, and records the baseline indicators on which the project will later be judged.
A senior engineer with twenty-five years' experience plans at district or state scale. The work is to prioritise hundreds of micro-watersheds with a defensible index, allocate budgets, set standard designs and schedules of rates, and review the quality of reports and works. Much of the job is institutional: keeping watershed committees functioning after withdrawal, resolving conflicts between upstream and downstream users, and linking watershed works with employment guarantee, irrigation and agriculture schemes. The senior engineer also checks claims in evaluations against the water budget, recognising when reported gains exceed what the rainfall could supply, and argues for basin-level planning where upstream harvesting is reducing inflows to downstream tanks and reservoirs.
A researcher with long field experience works on the questions programmes cannot answer by themselves: how much of an observed change in groundwater or yield is caused by treatment rather than rainfall, how watershed development at scale alters basin water balance and downstream flows, how morphometric and sediment-yield indices compare with measured sediment yield, and which institutional arrangements make benefits last. Methods include paired-watershed experiments, hydrological models calibrated on gauged catchments, remote sensing time series, and household panel surveys. The researcher also advises governments on revising guidelines and on the design of monitoring systems, and trains the next generation of engineers to combine hydrology with social analysis.
This manual does not give the hydrologic methods (rainfall analysis, runoff estimation, unit hydrographs), which are in Watershed Hydrology, or the structural design of bunds, terraces, spillways, ponds and dams, which is in Soil and Water Conservation Engineering; GIS and remote sensing procedures are only outlined and belong to the remote sensing course. Slope thresholds for land capability classes, weightage values and delivery ratios for the sediment yield index, beneficiary contribution rates and cost norms under current Indian guidelines, and state-specific scheme details are not reproduced because they are set by national manuals and change between guideline revisions; use the current official documents. Watershed models (for example SWAT), groundwater modelling and detailed evaluation statistics are not covered. Programme descriptions outside India are brief and do not give current budgets or institutional arrangements.
Original practice questions written to the syllabus and pattern of the exams named in each question's examTags. They are not past papers and do not reproduce any board's questions.
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