Evaluation and design of runoff storage systems for supplemental irrigation: Laikipia District case study

Abstract

An inventory of the existing runoff harvesting systems for crop production was carried out in Rugutu location of Laikipia district. An evaluation of rainwater storage system was also performed in two of the water pans. This was carried out using the water balance approach. It was realized that the major hindrance to the adoption of runoff storage water harvesting systems was the excessive water loss through evaporation and seepage. The evaporation losses ranged between 0.1 and 0.3m3/day while seepage losses ranged between 0.03 and O.3m3/day which accounted for 30 to 50% of the total storage. Seepage water losses increased with an increase in water level. The water pans are neither lined nor covered with a roof and therefore most of the stored water is lost through seepage and evaporation. The historical rainfall record is bounded to the right by zeros. Frequency analysis was carried out analytically using the mixed distribution to determine the design rainfall at different reliabilities. It would be misleading to design systems based on the means as it was observed that the' mean rainfall values could only be expected at 30 to 40% reliability levels. The failure rate of a design based on the means would be high. The Markov model was used to determine the conditional and unconditional probabilities of wet and dry decades as well as the occurrences of different lengths of wet and dry spells. The start of the growing season was on the io" decade during the long rains and on the 29th decade during the short rains. The length of the growing period during the long and short rains was 80 days and 60 days respectively. Rainfall analysis shows that during the growing seasons, the probability of having a dry spell longer than 2 decades was higher than 50%. The dry spells are preceded by wet spells and runoff water harvesting for supplemental irrigation is then vital to avert the intra seasonal dry spells. The construction of water pans was funded by NGOs operating in the area. They did not take into consideration the catchment size available or the size of farms to be irrigated when deciding on the storage sizes. This has led to the under design of the water pans. The water pans had capacities ranging from 30-40m3 while the catchment areas ranged from 4000 to 10000m2. Mass curve analysis was used to determine the catchment area and storage capacity at different irrigated areas and rainfall reliability. Design tables have consequently been developed and they will help farmers in selection of the proper storage system. A V III farmer planning to practice supplemental irrigation on a 250m2 kitchen garden with a crop planted at the onset of the long and short rains and at a reliability of 80% would require a water pan with '40.2m3 and 50.3m3 storage capacity and a runoff catchment area of 4290m2 and 7499m2 respectively. The choice of the design reliability level selected would depend on the individual farmers' financial status. Soil moisture balance was carried out for different planting decades to determine when and how much water should be applied. The results have been presented in tabular form and will help a farmer when scheduling water applications. A crop planted at the onset of the long and short rains at rainfall reliability of 80% would require a total of 228mm and 232mm of supplemental irrigation water respectively. Water stored may not be enough to meet the full crop water requirement and deficit irrigation may be considered. In this case, crop water deficits should be avoided when the crop is at its sensitive growth stages. For cabbage crop this occurs during the head formation stage. Farmers within t he study area practice different methods 0 f runoff conservation for crop production. They mainly practice in-situ water conservation while the adoption rate of the runoff storage systems is low. Out of the 11 homesteads visited, only 3 of the homesteads had operating water pans. There is however a great potential of improving the livelihood of people living in these area through runoff storage systems for crop production.