In Situ Water Harvesting Techniques for Soil Conservation

Across India’s rainfed farmlands, a significant portion of every monsoon rain is lost to surface runoff-carrying away not just water, but the fertile topsoil farmers depend on. In situ water harvesting addresses this directly. Unlike methods that collect and transport runoff elsewhere, in situ techniques work right where rain falls, slowing it down, holding it in place, and letting it seep into the soil. The result is better soil moisture, reduced erosion, and improved groundwater levels-all without complex infrastructure. Five key techniques drive this approach: contour bunding, bench terracing, contour trenching, nala bunding, and contour vegetative barriers.

What is in situ water harvesting? 🔗

In situ water harvesting is the practice of capturing and retaining rainwater at the point where it falls, rather than allowing it to drain away. According to the FAO, water harvesting techniques that store water as soil moisture work by preventing or significantly reducing runoff using structures that encourage infiltration-increasing the proportion of rainfall entering soil storage, where plants can use it directly. This approach is especially valuable in rainfed agriculture, where crops depend entirely on monsoon rains and have no irrigation backup.

Research on soil and water conservation in rainfed areas notes that engineering measures-including bunding, terracing, and trenching-are typically employed when land slopes exceed 2% and agronomic methods alone are insufficient to prevent erosion. In situ techniques are the first line of defense, working with the natural shape of the land rather than against it.

Contour bunding 🔗

Contour bunding involves constructing low earthen embankments along the contour lines of sloping land-lines of equal elevation. These bunds interrupt downhill water flow, creating small temporary storage areas where runoff slows and gradually soaks into the soil. FAO’s water harvesting guidelines highlight that improved alignment of bunds along the contour makes the technique considerably more effective, and that stone bunds in particular require less maintenance than earthen ones because they act as semi-permeable filters that spread runoff more evenly.

Where contour bunding works best 🔗

Contour bunds are most effective on land with slopes under 6% and annual rainfall below 600 mm, particularly in red soils and sandy loams that can absorb the impounded water well. In deep black cotton soils, the technique can be problematic-cracks that form during dry periods may cause water to seep through and breach the bund. Successful construction requires spillways to manage excess flow during heavy rain, and bund surfaces are often stabilized with grasses or legumes to prevent erosion of the structure itself.

The benefits are well-documented. A 2025 study in Frontiers in Sustainable Food Systems found that in the Mahi Ravine Watershed in Gujarat, contour bunding and bench terracing together greatly decreased soil erosion and runoff in sapota orchards. In Maharashtra’s semi-arid regions, farmers using contour bunding have reported measurable improvements in soil moisture and crop yields, particularly for rainfed millets and pulses.

Bench terracing 🔗

Bench terracing transforms steep slopes into a series of flat, step-like platforms. Workers cut horizontally into the hillside, and the excavated soil is used to build retaining walls or earthen risers on the downslope edge. Each bench has a slight inward gradient so water doesn’t immediately run off the edge but instead collects on the platform, giving it time to infiltrate.

Design and suitable conditions 🔗

FAO’s bench terracing guidelines specify that this technique is suited to slopes ranging from 7 to 25 degrees for hand-built terraces. Two main types serve different purposes: irrigation or level bench terraces for crops like rice that require water impounding, and upland bench terraces for rainfed crops, which are generally sloped slightly for drainage. In humid regions, a reverse-sloped (inward-facing) design retains more water; in arid or semi-arid regions, outward-sloped terraces allow excess water to drain safely.

Beyond water retention, bench terracing eliminates the steep gradients that cause sheet and rill erosion, protecting topsoil that would otherwise wash away. In Himachal Pradesh’s hilly terrain, bench terracing has converted steep slopes into productive agricultural land-benefiting orchard crops like apples while significantly reducing soil loss. The main limitation is cost: bench terracing requires substantial initial labor and investment, making it most viable where high-value crops justify the expenditure.

Contour trenching 🔗

Contour trenching involves excavating continuous trenches along the contour lines of sloping land. Rather than raising a barrier above the surface like a bund, trenches work below it-acting as linear reservoirs that intercept runoff and hold it until it infiltrates into the surrounding soil. The excavated soil is typically placed on the downslope side of the trench, forming a small embankment that adds to the water-holding capacity.

Design specifications and applications 🔗

Trenches are generally 0.5 to 1 meter deep and 1 to 2 meters wide, spaced according to slope gradient and soil type-steeper slopes require closer spacing. The technique is well-suited to slopes between 5-20% and is particularly effective in high-rainfall areas where surface runoff volumes are substantial even during short storm events. It is also commonly used in agroforestry systems, where trees planted alongside the trenches benefit from the additional soil moisture while their roots help stabilize the trench walls. One important maintenance task is periodic de-silting, as sediment accumulates in trenches over time and reduces their storage capacity.

Nala bunding 🔗

Across India’s agricultural landscapes, seasonal drainage channels-locally called nalas-carry large volumes of runoff during the monsoon but remain dry for most of the year. Nala bunding puts these natural channels to work by constructing low barriers across them at regular intervals, creating a series of small temporary reservoirs that slow the water, encourage infiltration, and recharge groundwater.

Construction and benefits 🔗

Nala bunds are built using locally available materials-earth, stone, or concrete-depending on the channel size and expected flow. A minimum catchment area of 10 hectares and a nala depth of at least 1 meter are typically required for the structure to function effectively. Each bund must include a spillway to safely discharge excess water during heavy rainfall and prevent structural failure from overflow.

The groundwater recharge impact of nala bunding is among its most significant benefits. When stored water infiltrates slowly over a larger area, a portion percolates beyond the crop root zone and recharges underlying aquifers. In Karnataka’s undulating landscapes, nala bunding has proven effective at improving water availability for irrigation, leading to increased crop productivity and greater crop diversification. Nala bunds also moderate flood peaks during heavy rainfall by temporarily storing excess water and releasing it gradually, reducing the risk of downstream flooding.

Contour vegetative barriers 🔗

Contour vegetative barriers use living plants-grasses, shrubs, or trees-planted in strips along contour lines to slow runoff, trap sediment, and increase infiltration. Unlike physical structures, vegetative barriers do not dam water; they filter it. Water passes through the hedge at reduced velocity, depositing its sediment load behind the barrier and continuing downslope without erosive force. Over time, sediment accumulation behind the strips builds up natural terraces, which become permanent landscape features.

Vetiver grass: the most effective species 🔗

Vetiver grass (Chrysopogon zizanioides) is widely recognized as the most effective species for contour vegetative barriers. Native to India, it forms dense, erect clumps with roots that can penetrate more than a meter into the soil, binding soil firmly against rilling and gullying. Field trials under watershed development initiatives in Uttarakhand demonstrated that vetiver reduced surface runoff by 40-50% and soil erosion by up to 70%, while also enhancing soil organic matter and moisture retention in terraced farming systems. In Karnataka, farmers with vetiver hedges along contour lines report finding moist soil behind their barriers during dry spells when neighboring unprotected plots are parched.

Research published by the National Academies Press notes that vetiver hedges are also more forgiving than physical bunds-because they filter rather than dam water, an occasional gap in the hedge does not cause catastrophic failure the way a breach in an earthen bund would. From an economic standpoint, a comparative analysis published on IntechOpen found that vetiver systems are more cost-effective than both engineered structures and most other vegetative barriers, with costs roughly one-third those of conventional contour bunds.

Vetiver takes two to three growing seasons to establish a hedge dense enough to withstand heavy rain, but once established, it requires minimal maintenance and provides benefits for decades. Beyond soil and water conservation, the barriers supply fodder for livestock, biomass for composting, and habitat for beneficial insects-making them a genuinely multipurpose farm investment.

Choosing the right technique 🔗

No single in situ technique fits every situation. Selecting the right approach depends on slope gradient, soil type, rainfall pattern, and available resources. The FAO recommends integrating water harvesting structures in landscapes in a planned and systematic manner, creating a “water buffer” that reduces vulnerability to drought and seasonal rainfall variation.

As a general guide: gentle slopes (2-8%) suit contour bunding; steep terrain (15-35%) calls for bench terracing; high-rainfall slopes between 5-20% benefit from contour trenching; undulating landscapes with seasonal streams are ideal for nala bunding; and vegetative barriers work across a wide range of slopes, especially as a low-cost complement to other techniques. Many successful watershed programs combine two or more of these methods, layering physical structures with living barriers for maximum effectiveness. Combining vetiver barriers with trench-cum-bund systems, for instance, has been shown to deliver better runoff and soil loss control than either method alone.

What do you think? Given the variety of in situ water harvesting techniques available, which approach do you think is most practical for small and marginal farmers with limited resources? And how might the increasing unpredictability of monsoon rainfall change the way these techniques are prioritized in different agro-climatic zones?

References

1. https://www.fao.org/land-water/water/water-management/water-storage/en/
2. https://link.springer.com/chapter/10.1007/978-981-99-8425-1_15
3. https://www.fao.org/4/u3160e/u3160e07.htm
4. https://www.frontiersin.org/journals/sustainable-food-systems/articles/10.3389/fsufs.2025.1612949/full
5. https://www.fao.org/4/ad083e/ad083e07.htm
6. https://www.sciencedirect.com/topics/earth-and-planetary-sciences/contour-farming
7. https://link.springer.com/article/10.1007/s43621-025-02014-x
8. https://nap.nationalacademies.org/read/2077/chapter/6
9. https://www.intechopen.com/chapters/55730