Towards a Better Design of Micro-Catchment Water Harvesting Schemes. By Dave Best
Direct rainwater harvesting from micro-catchment schemes (within field catchment systems) • Overland flow harvested from a short catchment. 1-30 metres. • Concentration of surface runoff from a catchment into lower lying cropped area. • Ratio of catchment to cultivated area- from 1:1 to 20:1. • Offers a passive, low cost means of increasing the amount of water available to crops
Advantages over alternative irrigation schemes. • Systems relatively easy to maintain and operate. • Simple and easy to construct • Once constructed little maintenance is required. • Runoff water low salt content
Examples of micro-catchment water harvesting systems 1) Negarim micro-catchments 2)Contour bunds
3) Semi circular bunds 4). Contour ridges
Design Considerations • To improve catchment efficiency runoff can be increased, and infiltration and interception reduced. • Balance between the efficiency of a catchment in producing runoff and its susceptibility to erosion • Increasing runoff by clearing slopes of vegetation and compaction of surfaces may lead to greater erosion with longer slope lengths and steeper gradients.
Determination of Catchment yield. • Rainfall Characteristics. • average annual rainfall, peak rainfall intensity, and minimum expected annual precipitation. • Physiographic factors. • Infiltration Capacity.
Catchment : Cultivated area • Rainfall • Runoff • Crop water requirements.
Catchment : Cultivated area-calculations……. • Water Harvested = Catchment area x Design rainfall x Runoff coefficient x Efficiency factor.
Example. • Crop Water Requirement = 525 mm • Design Rainfall = 310 mm • Runoff Coefficient == 0.5 • Efficiency Factor == 0.75 C = 525 – 310 = 1.85 Ca 310 x 0.75 x 0.5
Surface treatments • Mechanical treatments (smoothing and compacting). • Colloidal dispersion methods (slaking, sodium). • Hydrophobic applications (water repellents). • Surface binding materials (cementing and sealing) • Surface covering (asphalt, rubber and plastic)
Experiment testing different techniques for catchment lining on a plantation of Jujube (Zizyphus mauritaina) • Shallow conical micro-catchments of 1.0m radius were constructed around the plant
The soil moisture depletion pattern in various treatments in a 90cm profile