Forestry & Forest Sciences · Forest Ecology & Biology · also Watershed Management, Forest Influences, Forest Hydrology
How forests affect the water cycle and how forested watersheds are managed for water yield, quality and soil protection. Engineering design of conservation structures is covered in the Agricultural Engineering track (watershed-hydrology, soil-water-conservation-engineering); this subject covers the forest side.
In the exams: IFoS Forestry Paper I Section B (watershed management); ICAR AICE-JRF 59 Unit 8; ICAR AIEEA-PG Group 09 (watershed management); Korea Forestry Engineer written subject Erosion Control Engineering
Forest hydrology is the study of how forests affect the movement and storage of water: how much rain reaches the ground, how much returns to the atmosphere, how much flows to streams and aquifers, and when. Watershed management applies this knowledge, together with soil conservation, to manage land and water in whole drainage basins for water supply, flood and erosion control, and the livelihoods of the people who live there. Foresters have long been custodians of the upland catchments that supply water to cities, farms and hydropower, and the protection of water was one of the earliest reasons for reserving forests.
Popular beliefs about forests and water are strong and sometimes wrong. Forests do protect soils, reduce erosion and usually deliver cleaner water; but they also use more water than most other vegetation, so planting trees on grassland generally reduces streamflow rather than increasing it, and forests cannot prevent the largest floods. A forester needs to know what the evidence shows, including the results of paired-catchment experiments that have tested these questions for more than a century.
This subject covers forest influences on climate, water and soil; interception, throughfall, stemflow and infiltration; evapotranspiration and water yield; the effects of harvesting, plantations and fire on streamflow; the watershed concept, delineation and prioritisation; erosion and sediment yield from forest land; vegetative and simple structural measures for watershed treatment; riparian buffers; integrated and participatory watershed programmes; and paired-catchment studies and hydrological monitoring. Two related subjects are cross-referred rather than repeated: the engineering hydrology of runoff estimation, hydrographs and design of conservation structures is in the Agricultural Engineering track's watershed-hydrology and soil-water-conservation-engineering subjects, and the ecology of forest ecosystems, including nutrient cycling, is in forest-ecology.
Watershed management is examined in Section B of Paper I of the Indian Forest Service forestry optional (forest soils, soil conservation and watershed management), Unit 8 of the ICAR AICE-JRF forestry syllabus, the ICAR AIEEA-PG forestry group, and as Erosion Control Engineering in Korea's Forestry Engineer qualification. Questions ask for the components of the forest water balance, the effects of forests on floods and water yield, the findings of catchment experiments, the stages of erosion, prioritisation methods, conservation measures and the features of watershed programmes.
Forest influences are the effects of forests on their environment beyond the site itself: on local and regional climate, on the quantity, timing and quality of water, and on soil. The idea is old; the protection of watersheds was a principal reason for creating forest reserves in many countries, and the study of forest influences was a recognised branch of forestry in the nineteenth and twentieth centuries. Modern forest hydrology has tested many traditional claims and refined them.
Forests modify microclimate: under a canopy, temperatures are lower by day and higher by night than in the open, humidity is higher and wind speed is lower. At larger scales, forests influence climate through their low albedo (they absorb more solar radiation than grassland or snow), their high evapotranspiration (which cools the surface and adds water vapour to the atmosphere) and their aerodynamic roughness. Large forests can recycle a substantial share of rainfall: in the Amazon, moisture evaporated from the forest is carried downwind and falls again as rain, including over agricultural regions far away, sometimes described as flying rivers. Large-scale deforestation can therefore reduce rainfall downwind, while local effects of forests on rainfall amount are generally small. Tropical montane cloud forests capture water from fog and cloud directly on their foliage (fog interception), adding to the water input beyond rainfall.
Compared with other land covers, forests generally: intercept more rainfall, which evaporates without reaching the ground; transpire more water through deep roots that reach water unavailable to grasses and crops; maintain high soil infiltration capacity, so that little overland flow occurs; and deliver water of high quality, with low sediment and nutrient loads. As a result, forested catchments usually produce less total streamflow than grassland or cropland catchments with the same rainfall, but with smaller storm peaks from moderate storms, better sustained flows in some settings and much cleaner water. The often-repeated claim that forests act as a sponge that stores wet-season rain and releases it in the dry season is only partly true: forest soils do store water and sustain baseflow, but the higher water use of forests can reduce dry-season flow, and the net effect depends on climate, soils and forest type.
Forests protect soil against erosion by intercepting raindrops, by the litter layer that absorbs the energy of falling drops and slows overland flow, by roots that bind soil and reinforce slopes against shallow landslides, and by high infiltration that limits runoff. Forest soils typically have high organic matter, good structure and many macropores from roots and soil fauna. Undisturbed forest therefore produces the lowest erosion of any land cover, and the most serious erosion from forest land usually comes from disturbance: roads, skid trails, clearing, burning and grazing.
When rain falls on a forest, part of it is caught by leaves, branches and stems and evaporates back to the atmosphere without reaching the ground; this is interception loss. The rest reaches the ground as throughfall, which falls directly through gaps or drips from the canopy, and stemflow, which runs down branches and trunks. The canopy water balance is: gross rainfall = throughfall + stemflow + interception loss. Net rainfall, the water reaching the forest floor, is throughfall plus stemflow. For example, if gross annual rainfall is 1200 mm, throughfall 900 mm and stemflow 30 mm, interception loss is 270 mm, or 22.5% of rainfall, and net rainfall is 930 mm.
Interception loss depends on the canopy's storage capacity (the amount of water leaves and branches can hold, related to leaf area and surface texture), the rainfall pattern (many small storms produce higher proportional losses than a few large ones, because the canopy is filled and emptied each time), and the evaporation rate during and after rain (higher in windy, rough canopies). Interception losses commonly range from about 10% to 40% of annual rainfall depending on forest type and climate, and are generally higher for evergreen conifer forests, with dense needle canopies that hold water all year, than for deciduous broadleaves, especially in winter. Snow interception by conifers also reduces snowpack. Litter on the forest floor intercepts a further small amount.
In mountain and high-latitude catchments, forests affect snow, which stores winter precipitation and releases it in spring and summer. Conifer canopies intercept snow, much of which sublimates back to the atmosphere, so snowpacks under dense forest are often smaller than in openings. Forest shade, however, slows melting, which can spread snowmelt over a longer period. Small openings and gaps in forests can accumulate more snow than either dense forest or large open areas, a principle sometimes used in managing water-yield forests in snow-dominated regions.
Stemflow is usually a small fraction of rainfall, often a few percent, but it can be larger in species with upward-angled branches and smooth bark, such as some eucalypts and beech, and it concentrates water and dissolved nutrients at the base of the stem, where it infiltrates deeply along roots. Rough, thick bark absorbs water and reduces stemflow.
Infiltration is the entry of water into the soil. Forest soils usually have high infiltration capacity because the litter layer protects the surface from sealing by raindrop impact, organic matter improves structure, and roots and soil animals create macropores. As a result, Hortonian overland flow, which occurs when rainfall intensity exceeds infiltration capacity, is rare in undisturbed forests. Instead, storm runoff from forested catchments is generated mainly by subsurface stormflow (lateral flow through soil and macropores) and by saturation overland flow on areas near streams that become saturated, as described by the variable source area concept developed by Hewlett and others: the area contributing quick runoff expands and contracts with wetness. Compaction by machinery, trampling by livestock, fire and road surfaces reduce infiltration and can cause overland flow and erosion even within forests.
Over a catchment, precipitation (P) is partitioned into evapotranspiration (ET), streamflow or runoff (Q) and changes in storage (ΔS) in soil, groundwater and snow: P = ET + Q + ΔS. Over a long period, or a water year starting and ending at similar storage, ΔS is close to zero and Q ≈ P − ET. For example, a catchment receiving 1500 mm of rain a year with ET of 900 mm yields about 600 mm of streamflow, a runoff coefficient (Q/P) of 0.4. Because ET is the largest loss in most forested catchments, anything that changes ET changes water yield.
Evapotranspiration includes transpiration from plants, evaporation of intercepted water from the canopy and evaporation from the soil and litter. Forests usually have higher ET than grasslands or crops for several reasons: higher interception loss; deeper roots that keep transpiring into dry periods; larger, rougher canopies that exchange heat and water vapour with the air more efficiently; lower albedo, so more energy is available; and, in evergreen forests, transpiration through the whole year. Analyses of catchments worldwide, such as those of Zhang and colleagues (2001), show that for a given annual rainfall, forested catchments have higher ET than grassland catchments, with the difference greatest in humid climates where water is not limiting.
Water yield is the total streamflow from a catchment over a period, usually expressed as depth (mm) or volume. Because forests use more water, catchments under forest generally yield less water than similar catchments under grass or crops, and converting forest to non-forest generally increases water yield, while converting grassland or cropland to forest reduces it. The size of the effect depends on rainfall, the proportion of the catchment changed, the species and age of the forest and the climate. Exceptions include cloud forests, where fog interception may exceed the additional water use, and very old forests compared with regrowth (see the next chapter).
The effect of forests on dry-season flow, which matters most to downstream users in monsoon climates, is more complicated than their effect on annual yield. Two opposing processes operate. Forest soils with high infiltration allow more rain to enter the ground and recharge groundwater, which sustains baseflow; on the other hand, deep-rooted forests continue to transpire through the dry season, drawing down soil water and groundwater that would otherwise feed streams. Which effect dominates depends on soils, rainfall pattern, rooting depth and the degree of soil degradation. Where land has been severely degraded and compacted, so that most rain runs off quickly, restoring tree and ground cover can improve infiltration enough to increase dry-season flow despite the trees' water use; this is the basis of the 'infiltration-evapotranspiration trade-off' hypothesis described by Bruijnzeel and others. Where soils are still in good condition, planting trees on grassland usually reduces dry-season flow. Simple claims that forests always increase or always decrease dry-season flow should therefore be avoided.
ET can be estimated as the residual of the catchment water balance (P − Q, over long periods), measured directly over stands by eddy covariance towers that record turbulent fluxes of water vapour, estimated for individual trees by sap flow sensors in stems, calculated from weather data using equations such as Penman-Monteith with parameters for forest canopies, and mapped over large areas from satellite data. Each method has assumptions and uncertainties; combining them gives the most reliable picture. The reference crop evapotranspiration used in irrigation engineering (see the Agricultural Engineering track) is a different concept from actual forest ET.
Many catchment experiments have measured the effect of cutting forest on streamflow. A widely cited review by Bosch and Hewlett (1982) of 94 catchment experiments concluded that reducing forest cover increases water yield and establishing forest decreases it, and that, on average, a 10% change in forest cover changes annual water yield by about 40 mm for conifer and eucalypt forests, about 25 mm for deciduous hardwoods and about 10 mm for scrub. Changes in cover of less than about 20% were often not detectable against natural variability. The increase after cutting is greatest in wet years and declines as vegetation regrows, typically over years to decades. Later reviews broadly confirmed these patterns, while showing that responses vary greatly with climate and with how the forest is cut and regenerates.
Harvesting can increase storm peak flows, especially for small and moderate storms, through reduced interception and ET (wetter soils before storms) and, more importantly, through roads and skid trails that compact soils, intercept subsurface flow and route water quickly to streams. For large, prolonged storms in large basins, when soils are saturated regardless of vegetation, the effect of forest cover on flood peaks is small. A review by FAO and the Center for International Forestry Research in 2005 cautioned that the role of forests in preventing major floods in large river basins had been overstated, while confirming their importance for small catchments, erosion control and water quality.
Planting fast-growing trees on grassland or scrubland reduces streamflow, sometimes substantially, because the trees use more water than the vegetation they replace. In South Africa, where plantations of pines and eucalypts in grassland catchments reduced streamflow, the National Water Act of 1998 classifies commercial afforestation as a streamflow reduction activity requiring a water use licence. Studies of eucalyptus plantations in parts of India, including the Nilgiris, raised similar concerns about water use, although the effect depends on rainfall, rooting depth and water table. In mountain regions, regrowth after fire or logging can also change water yield: in mountain ash forests of Victoria, Australia, young regrowth after fire uses more water than old-growth forest, so streamflow falls for decades after fire before slowly recovering, a pattern described by the Kuczera curve.
Fire removes vegetation and litter, reduces interception and ET, and can make soils water-repellent, all of which increase runoff. After severe fires, streamflow, flood peaks and sediment loads can rise sharply in the first rainy seasons, and debris flows can occur on steep slopes. Ash and nutrients can degrade water quality and harm aquatic life and water supplies. As vegetation recovers, these effects decline, but as the mountain ash example shows, longer-term changes in water yield can follow from changes in forest age structure. Post-fire stabilisation is treated in forest-fire-science.
A watershed (also called a catchment or drainage basin, though usage differs between countries) is the area of land that drains to a particular point on a stream, river or lake, bounded by a water divide (ridge line). It is a natural unit for managing land and water, because what happens upstream affects water and sediment downstream. Watersheds are nested: small watersheds combine into larger ones, from micro-watersheds of a few hundred hectares to river basins of thousands of square kilometres. In India, the All India Soil and Land Use Survey developed a hierarchical delineation and coding system of water resource regions, basins, catchments, sub-catchments, watersheds, sub-watersheds and micro-watersheds, and watershed development programmes commonly treat micro-watersheds as the unit of planning and community action.
A watershed is delineated by tracing the ridge line around the drainage area above an outlet, traditionally on topographic maps and now usually with digital elevation models in a geographic information system, which compute flow directions and accumulate drainage area automatically. Morphometric analysis describes watershed shape and drainage network. Stream order, in Strahler's system, assigns first order to unbranched headwater streams; where two streams of the same order join, the order increases by one, while the joining of a lower-order stream does not change the order. Drainage density is the total length of streams divided by the watershed area: a watershed of 30 km² with 45 km of streams has a drainage density of 1.5 km/km². High drainage density indicates impermeable soils, sparse vegetation or steep relief and is associated with rapid runoff and high erosion potential. The bifurcation ratio (number of streams of one order divided by the number of the next higher order), form factor, elongation and circularity ratios describe shape; compact, circular watersheds tend to produce sharper flood peaks than elongated ones.
Because resources are limited, watersheds must be prioritised for treatment. Prioritisation considers the severity of erosion and land degradation, the importance of downstream assets (reservoirs, irrigation systems, towns), the poverty and dependence of local people on land, the availability of community interest, and the potential for improvement. The All India Soil and Land Use Survey uses a Sediment Yield Index, calculated from the area of erosion intensity units weighted by erosion and delivery factors, to rank sub-watersheds above major reservoirs into priority classes for treatment, so that soil conservation in the catchments of river valley projects targets the areas contributing most sediment. Morphometric parameters, land use and land cover maps from remote sensing, slope and soil maps are combined in GIS for prioritisation in many studies.
Watershed boundaries rarely match administrative boundaries such as villages, districts and forest ranges, which complicates planning: a single watershed may include several villages and land under forest, revenue and private ownership. Effective watershed management therefore requires coordination among departments and communities, and programmes in India define project areas as clusters of micro-watersheds while working through village-level committees.
Erosion by water proceeds through several processes. Splash erosion begins when raindrops strike bare soil, detaching particles. Sheet erosion removes a thin, fairly uniform layer of soil by shallow overland flow, often unnoticed until much soil is lost. Rill erosion occurs when flow concentrates into small channels that can be removed by tillage. Gully erosion forms larger channels that cannot be removed by normal cultivation and can expand rapidly by headward cutting, as in the ravines of the Chambal and Yamuna valleys of India. Stream bank and channel erosion supply sediment from within streams. Mass movements, including landslides, debris flows and soil creep, move large volumes of soil on steep slopes, and are a major source of sediment in mountains such as the Himalaya.
Undisturbed forest produces very little surface erosion because of canopy and litter cover and high infiltration; its C factor in the Universal Soil Loss Equation (described, with its other factors, in the Agricultural Engineering track) is among the lowest of any land use. Sediment from forested land comes mainly from disturbance: roads and skid trails, which can produce most of the sediment from managed forests; landings and log-yarding areas; clear-felling and burning on steep slopes; overgrazing in forests; fire; and shifting cultivation with short fallows. Shallow landslides increase after forest clearing as root reinforcement decays, typically for several years until regrowth roots take over.
In steep mountain terrain such as the Himalaya, the Western Ghats and the north-eastern hills of India, mass movements can deliver more sediment than surface erosion. Forest cover reduces the frequency of shallow landslides, those within the root zone, through root reinforcement of the soil and by drying the soil through transpiration; after clear-felling, landslide frequency typically rises for several years as old roots decay before new roots develop. Forests have little influence on deep-seated landslides, whose failure planes lie below the root zone and which are controlled by geology, groundwater and earthquakes. Road construction is one of the most important human causes of landslides, through cutting the toe of slopes, loading slopes with fill and concentrating drainage. Slope protection in forestry therefore emphasises keeping forest cover on steep, unstable slopes, careful location and drainage of roads, and bioengineering on disturbed slopes, using live stakes, brush layers and grasses with simple structures.
Not all eroded soil leaves a watershed; much is deposited on footslopes, in hollows and on floodplains. Sediment yield is the amount of sediment passing the outlet of a watershed per unit area per year. The sediment delivery ratio is sediment yield divided by gross erosion within the watershed: if gross erosion is 20 t/ha/yr and the delivery ratio is 0.25, the sediment yield is 5 t/ha/yr. Delivery ratios generally decrease as watershed area increases, because larger watersheds have more opportunities for deposition. Sedimentation reduces the storage capacity and life of reservoirs, a major concern for river valley projects, and degrades aquatic habitat and water quality.
Erosion and sediment are measured by runoff plots with collecting tanks, erosion pins and profile meters on slopes and gullies, sediment sampling at stream gauging stations (suspended sediment concentration multiplied by discharge gives sediment load), surveys of reservoir sedimentation, and tracer methods such as fallout radionuclides. Models, from the empirical USLE and its revisions to process-based models, estimate erosion where measurements are lacking.
Watershed treatment generally follows a ridge-to-valley sequence: treating the upper slopes and ridges first, to reduce runoff and erosion at their source, then the middle slopes and agricultural land, and finally the drainage lines and valleys. Treating drainage lines without first treating the slopes above them often fails, because structures are filled with sediment or washed away. Measures combine vegetative methods, which are cheap and self-renewing, with simple structures where vegetation alone is insufficient. The engineering design of structures, including runoff estimation and spillway sizing, is covered in the Agricultural Engineering track's soil-water-conservation-engineering subject; this chapter describes the forestry side.
Vegetative measures include afforestation and reforestation of degraded slopes with suitable species, including multipurpose trees that provide fuel, fodder and other products to local people; closure of degraded areas to grazing and protection from fire, which often allows natural regeneration of grasses, shrubs and trees with little cost; planting grasses and legumes on bunds, terraces and gully banks; contour vegetative barriers of grasses such as vetiver (Chrysopogon zizanioides) or of shrubs planted in lines along the contour, which slow runoff, trap sediment and gradually form natural terraces; live hedges and live check dams of fast-growing, easily rooting species planted across small gullies; and agroforestry and silvipastoral systems on farmland. Vegetative measures improve soil organic matter and infiltration and provide products, but take time to establish and need protection.
Common simple structures on forest and common lands include contour trenches, continuous or staggered, dug along contours to catch runoff and sediment and store water for tree planting; contour bunds and stone walls on gentle slopes; gully plugs and check dams built of loose boulders, brushwood, earth or gabions (wire-mesh baskets filled with stones) across gullies to reduce flow velocity and trap sediment, with more permanent masonry structures at key points; diversion drains to carry runoff safely around vulnerable areas; percolation tanks and small ponds to store water and recharge groundwater; and spurs and retaining walls to protect stream banks. Loose boulder and brushwood check dams are cheap and use local materials and labour, but must be built in series, starting from the head of the gully, and maintained.
Choice of measures depends on slope, soil depth, rainfall, the type and stage of erosion, land use and tenure, availability of local materials and labour, and community priorities. Measures should be combined, for example trenches with tree planting, gully plugs with grass on the banks, so that structures provide immediate protection while vegetation becomes established and takes over. Maintenance is often the weakest link, so programmes increasingly emphasise community ownership and funds for maintenance.
The riparian zone is the land next to streams, rivers and lakes, where terrestrial and aquatic ecosystems interact. Riparian forests shade streams and keep water cool; stabilise banks with roots; filter sediment, nutrients and pesticides from runoff; supply leaves and insects that feed aquatic organisms; contribute large woody debris that creates pools and habitat in channels; and provide corridors for wildlife. Disturbance of riparian zones by logging, grazing, roads or cultivation increases stream temperature, sedimentation, bank erosion and nutrient loads, and degrades fish habitat.
A riparian buffer (streamside management zone) is a strip of land along a watercourse where activities such as clear-felling, machinery operation, chemical use and road building are prohibited or restricted. Buffer width is usually set by stream size, slope, soil erodibility and the value of the water resource or habitat; regulations and guidelines in different countries prescribe widths commonly from about 10 m to several tens of metres on each side, with wider buffers on larger streams, steeper slopes and important fisheries or water supplies. Some guidelines allow selective harvesting in the outer part of the buffer while protecting a no-harvest core. Buffers are among the most effective forestry best management practices for protecting water quality.
Best management practices (BMPs) for water in forest operations include planning roads to avoid streams and unstable slopes; minimising stream crossings and designing culverts and bridges to pass floods and fish; installing drainage on roads and skid trails (cross drains, outsloping, water bars) so that water is dispersed rather than concentrated; avoiding operations on saturated soils; locating landings away from streams; retaining buffers; revegetating disturbed areas promptly; and careful storage and use of fuels and chemicals. Monitoring of BMP implementation and effectiveness, practised in many US states and elsewhere, shows that well-implemented BMPs greatly reduce sediment from forest operations.
Where riparian vegetation has been lost, restoration may involve fencing to exclude livestock, planting native trees and shrubs suited to the moisture regime, stabilising eroding banks with bioengineering (live stakes, fascines, brush layers) and, where needed, reintroducing large wood. In India, riverbank and catchment plantations, protection of riparian strips in forest working plans and community-based restoration of springs in the Himalaya are examples of efforts to protect water at its source.
Early watershed programmes concentrated on soil conservation works in the catchments of reservoirs, with forest and agriculture departments building structures and planting trees, often with little involvement of local people. Experience showed that such works were often neglected or damaged once projects ended, because local people did not benefit from them or had not chosen them. Integrated watershed management therefore combines soil and water conservation with improvements in agriculture, livestock, forestry, water harvesting and livelihoods across the whole watershed, and participatory approaches involve communities in planning, implementation, cost-sharing and maintenance.
India has implemented watershed programmes on a very large scale. The Drought Prone Areas Programme and the Desert Development Programme began in the 1970s; the National Watershed Development Project for Rainfed Areas followed; and Common Guidelines for Watershed Development Projects were issued in 2008 to harmonise programmes. The Integrated Watershed Management Programme (IWMP), launched in 2009–10, was later merged into the Pradhan Mantri Krishi Sinchayee Yojana as its watershed development component. These programmes treat clusters of micro-watersheds through village watershed committees, with funds for natural resource management, production systems and livelihoods of landless and asset-poor households, and require contributions from beneficiaries towards maintenance. Earlier pioneering examples include Sukhomajri in Haryana, where in the 1970s and 1980s villagers protected degraded hills from grazing in return for shares of water from small dams that reduced siltation of the Sukhna Lake at Chandigarh, and Ralegan Siddhi in Maharashtra, where community-led water harvesting and social rules transformed a drought-prone village.
The Loess Plateau Watershed Rehabilitation Project in China, supported by the World Bank from 1994 to 2005, restored large areas of severely eroded land through terracing, afforestation, grazing bans and changes in land use, reducing sediment entering the Yellow River and improving incomes. Programmes in Ethiopia, Nepal and elsewhere have combined community-based soil and water conservation with livelihood improvements. Payments for watershed services (see forest-economics-valuation) link downstream water users with upstream land managers.
In the Himalaya and other mountain regions, springs are the main source of drinking water for many villages, and many have been reported to be drying or declining in flow. Spring-shed management identifies the recharge area of each spring, which may lie some distance away and is determined by geology as well as topography, and treats that area with measures that increase infiltration, such as contour trenches, protection from grazing and fire, and appropriate vegetation, while protecting the spring source itself. Hydrogeological mapping, community monitoring of spring discharge and participatory planning are central to this approach, which has been promoted by state governments and non-governmental organisations in several Himalayan states. It illustrates how watershed principles apply at small scales where groundwater pathways, not just surface drainage, govern water supply.
Reviews of watershed programmes identify common principles: community participation from planning onwards, with institutions that represent all groups including women and the landless; equity in sharing benefits, since water harvesting often benefits landowners downstream more than the poor whose grazing land is closed; secure tenure and clear rights to the products of treated land; a combination of conservation with productive and livelihood activities; technical quality of works; convergence between departments; long enough project periods and post-project maintenance arrangements; and monitoring of both biophysical and social outcomes.
The paired-catchment method is the classical experimental design for measuring the effect of land use change on water. Two (or more) similar, nearby catchments are monitored together during a calibration period, often several years, to establish a statistical relationship between their streamflows, usually by regression of the treatment catchment's flow on the control catchment's flow. One catchment is then treated (for example clear-felled, converted to grass or planted) while the other is left as a control. After treatment, the calibration relationship is used to predict what the treated catchment's flow would have been without treatment, from the control's observed flow; the difference between observed and predicted flow is the treatment effect. For example, if calibration gave Qtreated = 1.1 × Qcontrol + 20 (mm) and after treatment the control yielded 380 mm while the treated catchment yielded 500 mm, the predicted flow is 438 mm and the treatment effect is about 62 mm. The design controls for differences in climate between years, which would otherwise mask treatment effects.
The first paired-catchment experiment was at Wagon Wheel Gap in Colorado, United States, beginning in 1910. The Coweeta Hydrologic Laboratory in the southern Appalachians of North Carolina and the Hubbard Brook Experimental Forest in New Hampshire, both run by the US Forest Service, have produced long records showing the effects of clearing, conversion of hardwoods to pine, and road building on water yield, peak flows and nutrient losses; Hubbard Brook's experiments also showed large losses of nitrate after clear-felling and herbicide treatment. Catchment experiments in South Africa, New Zealand, Australia (including the mountain ash studies) and elsewhere underpin modern knowledge of forests and water. In India, hydrological research on catchments under different land uses has been conducted by institutions including the Central Soil and Water Conservation Research and Training Institute (now the ICAR Indian Institute of Soil and Water Conservation) and forest research institutes.
Hydrological monitoring measures the components of the water balance. Precipitation is measured with standard and recording rain gauges, distributed to represent the catchment. Streamflow is measured at gauging stations: small streams are often gauged with weirs (thin-plate V-notch weirs are common for low flows) or flumes (such as Parshall flumes), whose discharge is calculated from the water level (stage) using a standard equation; for a 90° V-notch weir, discharge in m³/s is approximately Q = 1.38 H^2.5, where H is the head in metres, so a head of 0.2 m gives about 0.025 m³/s. In larger rivers, discharge is measured by current meters or acoustic instruments at different stages to build a stage-discharge rating curve, and stage is recorded continuously. Sediment is sampled with suspended-sediment samplers and bedload traps, water quality by sampling and sensors, soil moisture by probes, groundwater by wells, and ET by the methods described earlier.
Paired-catchment experiments are expensive and slow, and their results apply most directly to small catchments. Modern studies complement them with long-term monitoring networks, nested catchment designs, isotopic tracers that distinguish old and new water, remote sensing of vegetation and ET, and process-based hydrological models calibrated on monitored catchments, which can explore scenarios of land use and climate change. Long, consistent records remain the most valuable asset in forest hydrology, so protecting research catchments and continuing measurements over decades is a priority.
Learn the canopy and catchment water balances and practise the simple calculations (interception loss, runoff coefficient, drainage density, delivery ratio, paired-catchment effect, V-notch discharge). Know the findings of Bosch and Hewlett and the limits of forests' effect on floods, the erosion processes, Strahler ordering, the ridge-to-valley sequence of measures and the history of Indian watershed programmes with examples such as Sukhomajri and Ralegan Siddhi. In descriptive answers, correct common myths with evidence and link forest management to water outcomes.
A forester five years in plans and implements watershed treatment on forest and common land: delineating micro-watersheds in GIS, prioritising areas, laying out contour trenches, check dams and plantations from ridge to valley, protecting riparian strips during harvesting, designing road drainage, and working with village watershed committees on microplans. Monitoring of rainfall, runoff and sediment at small sites helps evaluate works. Typical problems are poor maintenance, works placed in the wrong sequence and conflicts over closure of grazing land.
A senior forester shapes catchment management for water supply reservoirs, hydropower and cities: setting policies on plantations and water use, riparian protection standards, road and harvest practices, and programmes of catchment area treatment. The senior forester negotiates between water users, communities and departments, uses evidence from catchment studies to counter myths, and secures long-term funding and institutions for maintenance and monitoring.
A senior researcher studies forest-water interactions: ET and water yield under changing forests and climate, runoff generation, erosion and sediment, cloud forest hydrology, and the hydrological effects of fire and plantations; maintains long-term research catchments; develops models; and advises on water policy, payments for watershed services and climate adaptation.
The engineering design of conservation structures, runoff estimation methods (rational method, SCS curve number), hydrograph analysis and the factors of the USLE are covered in the Agricultural Engineering track and are not repeated. Buffer widths, programme budgets and cost norms vary by country and state and are not prescribed. The size classes of the AISLUS watershed hierarchy and the formula of the Sediment Yield Index are described only in outline. Numerical values such as interception percentages and Bosch and Hewlett's averages are typical figures with wide variation. Ecosystem processes are in forest-ecology and fire effects in forest-fire-science.
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