Spatial and Temporal Soil Water Availability in a Cultivated Inland Valley Wetland in Central Uganda

Abstract

Cultivation of inland valley wetlands contributes significantly to the livelihoods of rural communities in Uganda and East Africa at large. This adversely affects the stability of these ecosystems through alteration of the water retention. A study was conducted along a cultivated inland valley in Namulonge Lake Kyoga basin in Central Uganda. The main objective was to understand the spatial-temporal dynamics of soil water availability under the different wetland uses along three landscape positions. Specifically, the study was conducted to: (i) Determine the spatial variation of soil moisture across the major agricultural land uses and slope positions in the inland valley wetland; (ii) Investigate the temporal trend of soil moisture across major agricultural land uses and slope positions in the inland valley wetland; (iii) Quantify the amount of runoff generated from the major agricultural land uses across different landscape positions in the inland valley wetland. A Completely Randomized Block Design was established where the treatments included three land uses, that is upland annual crop (i.e., maize, beans, sweet potatoes land uses); Arrowroots and Fallow, established along three landscape positions as blocks (Fringe/Upper, Middle, Riparian/Lower) of the inland wetland valley. Daily soil moisture was measured using Frequency Domain Reflectometry sensors under the three land uses and along the hydrological positions at four soil depths 10, 20, 30 and 40 cm from March to September 2020. The model was calibrated using the daily soil moisture and groundwater level for each land use along the hydrological positions. Results indicated that soil moisture content under different land uses varied significantly along the landscape positions with soil depths (p<0.05). Along the riparian position, the land use (arrowroots-yam) at depths to 0-20 cm and the land use (upland crops) at depths 20-40 cm had highest value of moisture content. Along the middle position, the land under fallow at depths 0-20 cm and the land use arrowroots at depths 20-40 cm had highest value of moisture content. Along the fringe position, the land use upland crops at depths 0-40 cm had highest value of moisture content. Soil moisture content trend for both the rain and dry seasons were well reproduced by the model for the two top soil layers (0.5=R2≤0.90). Monthly (temporal) soil moisture content increased quadratically in the different land uses with seasonal trends higher under arrowroots than upland crops and fallow lands. Runoff varied significantly with land uses and landscape positions. The study demonstrated that there is significant influence of land use on soil moisture availability, run off and this effect varies with landscape position. The land use under arrowroots had the highest capacity to retain soil moisture for long compared to other land uses, hence the land use could be instrumental in building resilience to water stress compared to other land uses in the in-land valley.