Dryland farming and moisture conservation
Farming where rain is scarce and unreliable: defining drylands, capturing and keeping rainwater in the soil, crop choices, and a water-limited yield sum.
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Drylands cover around 40 per cent of the Earth's land surface and are home to some two billion people. Much of the world's sorghum, millet, pulses, wheat and livestock comes from land where crops depend entirely on rain that is too little, too irregular, or both. In these places, the main limit on yield is not fertiliser or variety but water: how much falls, how much enters the soil, how much stays there, and how efficiently the crop turns it into grain.
Dryland farming is the set of practices for making the most of scarce rainfall without irrigation. It is a core topic in agronomy courses and in examinations for regions with semi-arid climates, which exist on every continent.
Defining drylands
Drylands are defined by the balance between rainfall and the atmosphere's demand for water. The aridity index used by UNEP and the UN Convention to Combat Desertification is annual precipitation divided by potential evapotranspiration (P ⁄ PET):
| Zone | Aridity index (P ⁄ PET) |
|---|---|
| Hyper-arid | below 0.05 |
| Arid | 0.05–0.20 |
| Semi-arid | 0.20–0.50 |
| Dry subhumid | 0.50–0.65 |
Cropping without irrigation is mostly possible in the semi-arid and dry subhumid zones; arid zones are mainly rangeland. Rainfall in drylands is typically not only low but variable between years, erratic within the season (with dry spells at critical stages), and often falls in intense storms that run off rather than soak in.
Some textbooks also distinguish "dry farming" (very low rainfall, crops grown on stored moisture), "dryland farming" (moderate rainfall with frequent dry spells) and "rainfed farming" (adequate rainfall), with thresholds that vary between authors.
The water balance of a dryland field
Rain that falls on a field can:
- run off the surface, carrying soil with it;
- evaporate directly from the soil surface, especially from bare, wet soil after small showers;
- drain below the root zone;
- be transpired by weeds;
- be transpired by the crop, the only part that produces yield.
Dryland practices aim to shift as much water as possible into crop transpiration, and to make each millimetre transpired produce as much yield as possible.
Capture more of the rain
- In-situ water harvesting, holding rain where it falls:
- contour cultivation and contour bunds across the slope;
- ridges and furrows, tied ridges (furrows blocked at intervals) and basin tillage;
- planting pits, such as the zaï pits of the West African Sahel, which concentrate water and manure around each planting station, and half-moon micro-catchments;
- broad beds and furrows on heavy clay soils.
- Ex-situ water harvesting: runoff collected from a catchment into ponds or tanks for supplementary irrigation at critical stages.
- Improve infiltration: crop residues and mulch to prevent crusting, organic matter, reduced tillage, breaking surface crusts.
Keep the water in the soil
- Mulching with residues, which cuts evaporation and keeps the surface cool.
- Weed control, because weeds transpire water the crop needs; early weeding matters most.
- Fallowing: leaving land unsown for a season to store rain for the next crop. Classic in wheat systems of semi-arid regions, but inefficient: often only about a fifth to a third of the rain falling during a fallow is still in the soil at sowing, the rest lost to evaporation, and bare fallows erode. Residue-covered, weed-free (chemical or minimum-till) fallows store more.
- Conservation agriculture: permanent cover and little disturbance improve infiltration and cut evaporation.
Use the water well
- Crop choice: drought-tolerant crops such as sorghum, pearl millet, finger millet, cowpea, chickpea, pigeonpea, groundnut, sesame, barley and durum wheat; C4 cereals use less water per unit of dry matter in hot conditions.
- Short-duration varieties that complete their life cycle within the rainy season, and varieties that escape terminal drought.
- Lower plant populations and wider spacing in drier conditions, so each plant has enough soil water to finish grain fill.
- Timely sowing with the first adequate rains; dry seeding before rains in some systems.
- Balanced nutrition: well-fed crops use water more efficiently; fertiliser is often placed with the seed or applied in small doses, and topped up only when rains are good (response farming).
- Intercropping and mixed cropping, which spread risk.
- Contingency crop planning: alternative crops and varieties ready for late onset of rains or mid-season drought.
- Integrating trees and livestock, which use land and seasons crops cannot.
How much yield can the water support?
A practical benchmark for wheat in dry, winter-rainfall regions was proposed by R. J. French and J. E. Schultz in southern Australia in 1984:
Growing-season water is rain during the season plus plant-available water stored at sowing, in millimetres; 110 mm is an estimate of soil evaporation; 20 kg of grain per hectare per millimetre is the transpiration efficiency of a well-managed crop. Later work has refined these numbers, and other crops have their own, but the idea is widely used to set realistic yield targets and to measure the yield gap.
Setting a realistic yield target
A wheat crop receives 220 mm of rain in the growing season and starts with 80 mm of plant-available water stored in the soil.
- Growing-season water = 220 + 80 = 300 mm.
- Water-limited potential = 20 × (300 − 110) = 20 × 190 = 3,800 kg/ha.
If the farm actually harvests 2,300 kg/ha, the yield gap is 1,500 kg/ha, and water use efficiency is 2,300 ÷ 300 ≈ 7.7 kg/ha per mm. The farmer would then look for the causes: weeds using water, late sowing, nitrogen shortage, disease, or water lost to runoff. Planning fertiliser for 3,800 kg/ha in a season heading for 150 mm would, by the same formula, waste money: 20 × (150 − 110) = 800 kg/ha.
Key points
- Drylands (aridity index 0.05–0.65) cover about 40 per cent of the land; semi-arid and dry subhumid zones support rain-fed cropping.
- Dryland rain is low, variable, erratic and often intense; the aim is to turn more of it into crop transpiration.
- Capture: contour farming, tied ridges, planting pits, micro-catchments, water-harvesting ponds, better infiltration.
- Conserve: mulch, weed control, residue-covered fallows and conservation agriculture.
- Use well: drought-tolerant and short-duration crops, lower populations, timely sowing, balanced nutrition, intercropping, contingency plans.
- French–Schultz: potential wheat yield ≈ 20 × (growing-season water − 110) kg/ha.
What the exams ask
- School-leaving and entrance papers: characteristics of drylands, moisture conservation practices, and drought-tolerant crops.
- Degree and research-fellowship entrances: aridity index, in-situ and ex-situ water harvesting, fallow efficiency, contingency crop planning, and water use efficiency.
- Professional and policy examinations: designing dryland farming systems, watershed approaches, response farming, and setting yield targets from water supply.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Is fallowing a good way to store water?
It can raise the next crop's yield, but it is inefficient: most rain during a bare fallow evaporates, and bare soil erodes. Residue-covered, weed-free fallows store more water, and in many systems a legume or short crop pays better than a fallow.
Why use fewer plants in dry seasons?
A dense crop uses soil water quickly during vegetative growth and can run out before grain fill. Fewer plants leave water for the end of the season, when it matters most for yield.
What are zaï pits?
Small planting pits, often about 20 to 40 cm across and 10 to 20 cm deep, dug in crusted, degraded soils of the West African Sahel and filled with manure or compost. They catch runoff and concentrate water and nutrients around the crop, and have helped rehabilitate degraded land.
Sources
- French, R. J. and Schultz, J. E. "Water use efficiency of wheat in a Mediterranean-type environment. I. The relation between yield, water use and climate." Australian Journal of Agricultural Research 35, 743–764 (1984).
- Sadras, V. O. and Angus, J. F. "Benchmarking water-use efficiency of rainfed wheat in dry environments." Australian Journal of Agricultural Research 57, 847–856 (2006).
- UNCCD. Global Land Outlook, 2nd edition. United Nations Convention to Combat Desertification, 2022.
- Rockström, J. et al. "Managing water in rainfed agriculture: the need for a paradigm shift." Agricultural Water Management 97, 543–550 (2010).
- Reij, C., Scoones, I. and Toulmin, C. (eds) Sustaining the Soil: Indigenous Soil and Water Conservation in Africa. Earthscan, 1996.
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