Hydrological impact of intensive rice cultivation in the inland valley of Tossahou in Benin
Inland valleys are often characterized by high soil fertility and assured water availability which makes them suitable for rice production. Nevertheless, unsustainable intensive agricultural production bears the risk of soil degradation and may impact downstream water availability and water quality. The current study investigates water dynamics, water quality, and rice production in the Tossahou inland valley located in the north-western part of Benin which is used for rice production. Climate and hydrology (dynamics of soil moisture, groundwater, and discharge) were monitored over two rainy seasons and the dry season in between. Management practices were surveyed, prevailing agricultural land use as well as soil physico-chemical properties have been mapped, and rice yield was measured. One of our specific objectives was the assessment of the suitability of the inland valley for four different rice-based cropping systems: rainfed upland (RU), rainfed bunded (RB), cultivation under natural floods (NF) and irrigated cultivation (IC). The climatic as well as the landscape and soil requirements were evaluated for each cropping system based on the parameter method developed by Sys et al. (1991, 1993) and the FAO Guidelines for Land Evaluation (FAO 1976). Results show that 79.8% of the analyzed area is suitable for RU, 52.1% for IC, 17.8% for NF and 1.2 for RB. Maximum rice yield of 6t/ha was observed in the inland valley in spite of the low rate of fertilizer application. These high yields are caused by the natural soil fertility and low fertilizer inputs account for low nitrate concentration in the discharge. The analysis of the seasonal and spatial variations of the shallow groundwater table revealed a highly fluctuating water table with the lowest level occurring during the dry season. Contrary to expectations, groundwater in the dry season was only accessible at the upstream part in the valley bottom with a depth to the water table of about 1m.
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