Assessment of Runoff for Design of Farm Ponds for Irrigation in Maragua Watershed, Murang’a County, Kenya

Abstract

This study involved the assessment of runoff and design of three different farm ponds for harvesting runoff to supplement irrigation. The study was carried out in Maragua watershed in Murang'a County where farmers face the challenge of water shortage during the dry season and therefore require water for supplemental irrigation of their horticultural crops. This was achieved by; assessing the water requirement for crops within the watershed, assessment of the runoff from the agricultural fields and design for the storage volume of a farm pond for the generated runoff from the agricultural fields. In this research, CropWAT model was used to determine the crop water requirement, while the Soil Conservation Service – Curve Number Model using ArcGIS 10.1 software was used in the estimation of the runoff depth in mm. The value of runoff depth was converted to runoff volume which was then used to design for the water storage facility. AutoCAD 2019 was used to make technical drawings for the farm pond. The design of the farm pond was done based on different slopes; gentle slopes and steep slopes in accordance to Design manual 2015. The study results showed that ET0 varied from 3.01 to 5.10 mm/day and the effective rainfall varied from 8.0 to 154.4 mm. The ETc values for the garden pea, sweet pepper and tomato were 395.6, 460.1 and 432.7 mm per the growing season respectively. The irrigation requirements were 181.4, 216.6 and 187 mm per season for garden pea, tomato, and sweet pepper respectively. The results indicate increasing ETc throughout the growth stages which is high at the mid-season stage and starts to decrease slightly at the later stages. The results for runoff estimation demonstrated that the SCS-CN method by using satellite imagery data to estimate runoff is convenient and effective. The runoff volume calculated from the area and depth of runoff indicates that the crop land had the highest volume of runoff since it occupied the largest area among the three land use classes which was at 9,368,519.10 m3, 2,858,923.47 m3, 2,511,768.72 m3, for Kambirwa, Gituamba and Maragua ridge micro-watersheds respectively. This runoff come from the entire sub watersheds which will supply water to the farm ponds in the area. The values indicate that there is enough runoff to be harvested to the farm ponds. The design of farm ponds was done for each of the ponds surveyed in the different micro watersheds using the practice manual for small dams. The different capacities for the three farm ponds were; the design criteria was obtained for the three different farm ponds in slopes of 1%, 2% and 3%. the design criteria was able to achieve the water demand, evaporation and seepage losses , dead storage, average storage capacity, spillway channel size and slopes. The study recommends CropWAT model as a suitable decision support tool for policy makers and investors on irrigation and water resources in the region with regard to irrigation water management.