Runoff is that portion of precipitation, which makes its ways towards stream, channel, lake or ocean as surface flow. Mostly runoff refers to surface flow only. Runoff from rainfall is inevitable and cannot be completely arrested. In dry farming areas, rainfall often occurs at high intensity, which exceeds the infiltration rate and causes runoff.
Also, when quantity of rainfall exceeds the water holding capacity of soils, runoff has to take place. In certain instances, surface characteristics of soils also cause runoff. Usually, under unchecked conditions, about 40% of rainfall may be lost as runoff. Even if moisture conservation practices are adopted, about 10–15% of rainfall in black soils and about 20% of rainfall in red soils is lost as runoff.
The amount of such runoff varies with rainfall intensity, soil physical properties, soil surface characters, slope, vegetation cover and cultural practices. Runoff water, if not checked, flows out and is wasted, causing soil erosion. It can be guided, collected and recycled to augment water availability to rainfed crops. The collection, storage and recycling of runoff water constitute the process of water harvesting.
Water harvesting can be viewed from two situations. First is a case of normal rainfall with high intensity on a few rainy days causing runoff. This runoff can be guided and collected in storage structures called farm ponds and reused for supplemental irrigation to crops suffering from moisture stress. This is termed as macro watershed approach or macro catchment water harvesting. In the second instance, total rainfall is less and soil storage is inadequate for supporting crop growth.
Here part of the land is left barren and uncultivated. This is known as donor area and is treated in such a way as to increase runoff from rainfall. The runoff from the donor strip is directed towards the lower adjacent strip to increase soil moisture storage there. This strip is used for raising crops. This is called as micro watershed approach or micro catchment water harvesting.
(i) Water harvesting through farm ponds: The collection of rainwater and storing in big farm ponds is not a new concept in India. It is in vogue since early days in the form of tanks. Farm ponds are small storage structures constructed at the lowest point of a farm to collect and store runoff water.
Runoff from various parts of the catchment area is properly guided through grassed waterways into the farm pond. The following points need to be considered while constructing farm ponds.
• Deep heavy soils with low permeability are better suited for farm pond technology than shallow light soils with high permeability.
But, ironically, the usefulness of farm pond is more felt in light soils with low water storage capacity.
• Farm pond has to be constructed at the lowest point of the farm to collect runoff water from the entire farm area.
• Size of farm pond depends on rainfall quantity, soil type, area of catchment (farm size) and estimated runoff.
• Provisions for arresting soil inflow into the pond at the inlet point and a weir, for draining excess water when pond is full have to be made.
• Runoff has to be guided to the farm pond through grassed waterways.
• Water loss through seepage and, evaporation has to be checked. Seepage loss, can be reduced by lining the sides and bottom with soil + sand + cement or soil + cow dung + straw, spraying sodium chloride or sodium carbonate on the surface. Evaporation loss can be reduced by floating materials to prevent direct exposure of water surface, changing the shape of the pond to provide more depth rather than surface area (circular instead of rectangular).
Harvested water can be used for protective irrigation to crops at critical stages. Since runoff is properly guided through grassed waterways, erosion is checked. Earth excavated from ponds can be used for bunding and leveling of fields. Stored water can be used as drinking water for humans and animals, for spraying operations and for fish rearing. High value tree crops can be raised near farm ponds with protective irrigation. A chain of farm ponds can recharge ground water in the region.
(ii) Water harvesting under deficit rainfall: The situation here is that the seasonal rainfall quantity by itself is not sufficient to support a crop till maturity. Therefore, runoff of rainfall from a part of the land left uncultivated is directed to an adjacent strip, which alone is used for cropping. In this strip (run-on strip/recipient area), the rainfall falling on its surface is supplemented by runoff directed from the other strip of land (donor area/runoff strip) and total water supply available is increased to facilitate cropping.
This can be accomplished by the following practices. A portion of the field in the upper reach is left uncultivated. It is shaped or treated to increase runoff. This can be accomplished by covering the surface with polythene films or by water proofing it by spraying sodium carbonate or water repellant materials like silicone/asphalt or by shaping the land into a sloping, clear, smooth, compact surface to increase runoff. Runoff from this donor strip is guided to a smaller, strip on the lower reach to increase soil storage and to raise crops.
The proportion of ‘donor area’ to cropped area depends on rainfall quantity, duration of rainfall, soil properties and crop characters. In the cropped area, land is shaped to conserve moisture. Acceptability of this method is however limited in regions where pressure on land does not permit leaving a large area barren for runoff harvesting.
Creating micro relief in cultivated field between seed rows to direct rainwater to crop root zone is another approach. Here, small alternate strips of land of suitable width are left without cropping. These un-cropped strips are ridged up and compacted or shaped to slope towards seed rows to increase runoff, which will flow towards cropped strip.
The relative width of runoff strip and cropped strip varies from 2:1 to 4:1 depending on rainfall. Land shaping through raised ridges between crop rows, planting in shallow ditch or trench, formation of slopping beds towards tree trunk, saucer shaped basins around trees, semicircular or crescent shaped basins on the downward slope around trees etc., come under this category. The micro watershed methods are also termed as inter-row water harvesting or inter-plot water harvesting.
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