Quantifying rainfall-runoff relationships on selected benchmark ecotopes in Ethiopia: a primary step in water harvesting research

Abstract

Large areas of cultivated land in Ethiopia frequently suffer from drought, causing low crop yields and food insecurity. It was hypothesized that it may be possible to alleviate this problem by employing infield rain water harvesting (IRWH). Three representative semi-arid ecotopes in the Rift Valley were selected to test this hypothesis. They were the Melkassa Hypo Calcic Regosol, The Dera Calcic Fluvic Regosol and the Mieso Hypo Calcic Vertisol. The climate, topography and soils of the ecotopes were characterized in detail. Rainfall runoff studies were carried out over two rain seasons on replicated plots on these ecotopes comparing two soil surface treatments. They were conventional tillage (CT), simulating the initially fairly rough surface which results after normal tillage; and no tillage (NT), simulating the flat crusted surface expected on the runoff strip of the IRWH system. Rainfall amounts, rainfall intensity at one minute intervals, and runoff, were measured for each storm during the two rain seasons on each ecotope. The results were used to calibrate and validate the Morin & Cluff runoff model in order to enhance the extrapolation capability of the study results to other similar ecotopes. The study yielded the following useful results. · The rainfall pattern on all the ecotopes was characterized by occasional storms with fairly high amounts and high intensities (Pi) which greatly exceeded the final infiltration rates of the soil, causing a high proportion of the rain (P) to runoff (R), i.e. producing a high R/P ratio. For the NT treatment final overall R/P values for the two seasons on the Melkassa, Dera and Mieso ecotopes were 0.45, 0.52 and 0.32, respectively. These high values indicate that IRWH should produce a significant increase in yield due to its ability to reduce R to zero while concentrating the runoff in the basin area and increasing the water available for transpiration and therefore increasing yield. · Because of the textural and mineralogical properties of the topsoils, particularly the two Regosols soils; they disperse and form crusts easily when impacted by high intensity rain. The result was that after cultivation at the start of the rain season the surface of the CT treatment soon became very similar to that of the NT treatment. Accordingly no significant difference was found between the runoff from the NT and CT plots on the Melkassa Regosol and Dera Regosol. There was, however, a significant difference in this respect on the Mieso Vertisol with a more stable surface. · Runoff prediction in all the ecotopes were well done by the M & C model. · Two separate strategies were developed to estimate the maize yield increase that could be expected on the Melkassa Regosol by employing IRWH. From the nearby Melkassa Research Station it was possible to obtain maize yields for 15 seasons (1989-2003). These were used together with climate data, the CROPWAT model, and the runoff measurements, to estimate the benefit of IRWH. The two strategies produced yield increase estimates of 33% and 40% compared to CT.