Experimental study of water transfer induced by irrigation over a plot in the middle of the Senegal river valley
Whereas irrigation appears to be the main approach to satisfy the growing worldwide demand for cereal, soil and water degradation continues to be an on-going problem in agriculture development. One of the main four concerns of the regional group for research on Sahelian irrigated systems (including Mali, Mauritania, Niger and Senegal) is soil degradation in the middle Senegal River valley. These soils are subject to various forms of degradation, mainly from salinisation and/or alkalinisation. These degradation processes are strongly correlated with water table dynamics, with water level fluctuations being significant. Therefore, to guarantee sustainable development of irrigated agriculture in the area, irrigation must be coupled with complete and permanent monitoring of soil and water quality. In this paper, we present a complete study concerning water transfer in irrigated plots and its effects on the groundwater table. The experimental site is located in the Podor region, at 16°.37'N, 14°.52'W in the Donaye irrigated area. The surface area is about 50 ha. Water supply is assured by filling a main channel using a group of pumps on the Doué River. Irrigation of the parcels is performed with siphons from this channel. The experimental plot of 0.33 ha is used for an underwater rice crop. One or two rice crop production harvests are made every year, with the decision been taken by the farmers. There is no drainage system in the area. The experimental plot was equipped with 8 piezometers located along a stream line. One is situated between the plot and the river in order to study the water movements caused by the exchange between the groundwater and the water in the river. Three are situated in the plot, one close to the Doué River, one in the middle of the plot and one near a dam, which is the opposite boundary compared to the river. Four other piezometers are located beyond the dam to estimate groundwater input and output at this boundary. Five tensiometers at 20, 40, 60, 95 and 135 cm depth are placed close to the piezometers located in the plot. Four water content profiles were measured during the irrigation period at depths 20, 40, 60 95 and 135 cm and the irrigation was performed over 84 days. The water table level variations at 2 meters were recorded. The groundwater inflow decreased during the first part of the irrigation period due to infiltration below the irrigated experimental plot. This inflow increased during a second period due to water level variations in the river close to this site. At the output, the head gradient did not vary appreciably and the outflow was assumed to be constant during the observation period. Moreover, this gradient is quite low and the flow rate is very low. The water content and pressure profiles clearly show the infiltration of water in the soil during irrigation. The head gradients show the water movement in the unsaturated soil during infiltration and evaporation. Upward flow due to evaporation is observed at about 10 days after irrigation. The last measured water profile (76 days after the end of irrigation) shows that evaporation modifies the water content profile until at least a depth of 120 cm. The water balance during the irrigation period showed that the input due to irrigation and precipitation was equal to 4150 m3. The evapotranspiration output was estimated to be 2370 m3. Groundwater exchange at the downstream boundary can be neglected during the duration of irrigation (84 days), since the average hydraulic gradient remained low (less than 0.8 %) and the hydraulic conductivity of the aquifer was not important (about 250 cm/day). It was assumed that the water storage quantified with the rise of the groundwater level was equal to the difference between surface input and evapotranspiration (more than 40 % of the contributions), leading to a 27 % change in water content, which is quite reasonable for this type of soil (clay). The stored water was then recovered by evapotranspiration and groundwater outflow at the plot boundaries. We are away from a reasonable irrigation that would reduce the used water quantity and decrease the risk of soil degradation. This study allows a better understanding of the water dynamics in the experimental plot, which includes soil saturation, recharge processes, and exchanges between the aquifer and the river.