An evaluation and modelling of rainwater conservation and utilization: case studies of Sipili, Kenya and Mijjwala, Uganda

Abstract

Rainwater harvesting potential is not realised due to poor design, construction and operation ~f water harvesting systems. A survey was conducted in Sipili, Kenya and Mijjwala, Uganda to characterize and diagnose the existing Rainwater Catchment Systems in use and also to establish how rainwater was utilized. Roof and Ground Catchment Systems were documented. The harvested rainwater in Sipili and Mijjwala was stored in surface or subsurface tanks and open surface ponds. The major problem identified with the storage tanks was leakage due to cracked walls attributed to inadequate construction materials or poor workmanship. Storage tanks for·the Ground Catchment experienced siltation due to either lack of silt traps or their inefficiencies. Over 70 percent of the surveyed tanks were not roofed and experienced high evaporation losses. Open surface ponds, which were commonly used in Mijjwala were not lined and seepage loss was a problem. The amount of water used for domestic chores on farmsteads with adequate tanks, ranged from 65 to 110 litres in Sipili and 80 litres in Mijjwala for a family size of 8 members. When water was obtained from alternative sources, which could be rivers, boreholes or permanent open ponds, the time taken ranged from 0.78 to 1.56.hours in Sipili and 0.87 to 1.90 hours in Mijjwala. Rainfall analysis for Rumuruti, in the vicinity of Sipili indicated that rainfall onset occurred during the ro" decade of the year. A dry spell of 50 to 60 days during the growing season occurred. a month after the onset of rainfall resulting in a poor crop harvest for rainfed ,agriculture. The annual rainfall that occurred at 50, 60, 67, and 80 percent probability levels were; 379.1, 249,182.5 and 73.8 mm respectively. The expected rainfall amount at 67 percent probability level was used for design computations. Mathematical relationships for the designed Ground Catchment System reservoir were developed showing: water depth (ht) as a function of water volume (Vin) in the reservoir; exposed area (Ae) as a function of water depth (ht) and wetted area (Aw) as a function of water depth (ht). The relationships were utilized in determining evaporation and seepage losses from the reservoir. A computer model was developed based on the mass balance equation to facilitate simulation of water utilization from different volume reservoirs. Each storage capacity reservoir was meant to provide water for domestic use, watering two dairy cows and . supplemental irrigation of a cabbage crop. The maximum cropped areas for a cabbage crop under supplemental irrigation for planting done at anyone preferred decade up to crop maturity, for Sipili area, were determined for tank capacities of 50, 100, 150, 200, and 250 m3 The analysis showed that the optimum cropped area for each tank size could be obtained when planting ~as done on the 14th decade. The maximum annual harvest of a cabbage crop could be achieved when planting was done on the 2nd 14th and 26th decade for 50, 100 and 150 mJ tanks and on the 1'r, 13th and zs" decade for 200 and 250 m3 tanks. Such, results would give farmers in Sipili an opportunity to strategies for higher market pr.ices of the cabbage crop or the biggest crop harvest. The same was applicable to Mijjwala area provided the rainfall and evaporation data were available. The benefit-cost analysis with alSO 111 3 tank indicated that rainwater harvesting technology was a feasible venture.