| dc.description.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. | en |
