figure 2. Photo of irrigation canal which transfers water from the river to the irrigated fields. (click to enlarge) |
Irrigation water is transferred from the river through canals (Figure 2). One of hte primary crops is soft peppers which are flood irrigated (Figure 3). Profiles in irrigated fields (Figure 2) show variable matric potential and chloride inventories (24 to 77 kg/ha/m), attributed on the basis of resistivity logging to changes in clay content. EM-31 mapping showed resistivity contrasts of two orders of magnitude, suggesting that at the scale of the 0.5 km2
survey, ~1/3 of the area had clay layers at depth that hinder drainage.
A savannah site that was cleared only ~1 yr prior to sampling, but had not been cultivated, showed high matric potential (mean -2.5 m) and a low chloride inventory (12 kg/ha/yr) below 1 m depth, similar to a nearby rainfed agriculture profile, indicating a rapid response to land use conversion. Estimated recharge is 210 mm/yr.
High values of F and N in fertilizers, as well as atmospheric inputs and N-fixation in natural savannah, are likely explanations for the observed pattern related to land use. Low soil water concentrations in rain-fed profiles indicate that unsaturated zone salts may have reached the water table. |
