Modelling the hydrological impact of rice intensification in inland valleys in Benin (West Africa)

Abstract

The aim of this study is to assess the impact of climate change and rice intensification on water availability, water quality, and rice production. A spatial explicit approach was developed to determine suitable areas for rice production in the investigated inland valleys. The Soil Water Assessment Tool (SWAT) model is applied to simulate the hydrological behavior of inland valleys and their contributing watersheds considering water quantity and water quality. Three small headwater inland valleys were selected in the commune of Djougou in central Benin namely Kounga, Tossahou and Kpandouga. Kounga is characterized by the highest proportion of agricultural land use, followed by Tossahou while Kpandouga is dominated by natural vegetation and has the smallest proportion of cultivated areas. The watersheds areas are small than 5 km² and do belong to the Upper Ouémé catchment in Benin. <br /> For modelling purpose, soil and land use maps were generated for each inland valley watersheds. In addition to hydrological observations of shallow groundwater levels and streamflow, surface water quality was determined using weekly collected water samples at the outlets of the watersheds. In a first step, the HRU-based ArcSWAT2012 model was applied while in a second step, the grid-based SWATgrid model was used. Model results were analyzed concerning their capacity to capture water quantity and water quality processes within the selected watersheds. The satisfactory model performance obtained from calibration and validation of daily discharges was the base to simulate climate change, land use change, and management scenarios using the calibrated model parameters. The emission scenarios A1B and B1 of the Intergovernmental Panel on Climate Change Special Report on Emissions Scenarios (IPCC SRES) were combined with two land use scenarios defined at 25 % and 75 % of lowland conversion into rice fields. The management scenarios were developed based on the current rice cultivation system in the inland valleys and the rainfed-bunded cultivation system with and without fertilizers inputs. The scenarios were quantified and analyzed up to the year 2049 with a special focus on the period of 2040 to 2049.<br /> The suitability of the inland valley of Tossahou for rice production was investigated as a case study using a GIS-based approach that evaluates and combines biophysical factors such as climate, hydrology, soil and landscape, following the FAO parameter method and guidelines for land evaluation. Hence, soil and landscape suitability was assessed for three different rice cultivation systems: rainfed bunded, cultivation under natural flooding, and irrigated cultivation.<br /> The results revealed that more than 60 % of precipitation water is lost by evapotranspiration at all inland valley watersheds. Percolation is important in the Kpandouga watershed (28 % of precipitation) having the largest portion of natural vegetation, whereas surface and subsurface runoff reach the highest values in the Kounga watershed (105 and 92 mm). At all sites, nitrate loads are very low which is in accordance with the low fertilizer application rates. The water quality is not threatened by the occurring agricultural practices if a standard threshold of 10 mg/l NO3-N is applied. In future, the impacts of climate change will be more significant concerning streamflow than the impacts caused by land use change at all watersheds. Substantial reductions of streamflow by up to 35 %, 47 %, and 51 % are projected for Kpandouga, Tossahou and Kounga, respectively. However, an increasing development of the lowland into rice fields under the current cultivation system will compensate the climatic effect on streamflow by up to 15 % at Kpandouga but will slightly enhance the effect by up to 2 % at Kounga and up to 8 % at Tossahou. Changes to a rainfed-bunded cultivation system will have no significant impact on water availability downstream. The suitability assessment of the inland valley of Tossahou for rice production especially indicated that 52% of the inland valley is suitable for irrigated cultivation, 18% for cultivation under natural flood and 1.2% for rainfed bunded rice. Besides precipitation, an increase of temperature causes an increase in potential evapotranspiration which is a limiting factor for all cultivation systems. Flooding was the most limiting factor for cultivation under natural flood while irrigated and rainfed-bunded cultivation systems were mostly limited by steep slopes and soil texture respectively. However, the results revealed that the social and economic environment restrict the yields more than the biophysical properties of the inland valleys. <br /> In all watersheds, the temporal pattern of precipitation strongly impacts the streamflow dynamic. However, the combined effect of topography, soil properties, land use, and shallow groundwater dynamics also determines the variation in runoff, which is highest in Kounga, followed by Tossahou, and lowest in Kpandouga. As the system is water limited and not energy limited, the prevalence of water scarcity within the inland valleys is projected in long term due to the expected reductions in rainfall under climate change. Moreover, the altering effect of changes in land use on hydrologic processes within the watersheds will have no substantial impact on streamflow downstream. Although the uncertainties and limitations encountered in modelling, the strong performance of the SWAT model in small watersheds has been confirmed. Thus, the results achieved in this study can be used in spatial planning for sustainable development of rice cultivation with limited environmental impact on water resources in inland valley landscapes. Additionally, the intensification of rice on areas of favorable conditions will foster an optimized production if the social and economic constraints as the access to credit, the subsidies acquisition, and the access to market are overcome.