A water balance approach for investigating precipitation capture and water storage of conservation practices in Mali

Abstract

Any effort to improve agricultural production in the Sahel region of Africa must consider wateravailability. This is especially true of the rain fed crop productions systems that often face waterstress resulting from agricultural droughts (short but critical periods during the growing season).The Amenagement en courbes de niveau or ACN practice holds promise to increase cropyields, sequester more carbon, and increase local groundwater recharge. The practice isbasically contour farming, but the soil is tilled in a manner that permanent ridges are formedalong the contour with valleys between the ridges (see Figure 1). Field studies have shown ACNincreases crop yield in excess of 50% for millet, sorghum and maize (Gigou et al. 1997). Also,increases of soil organic carbon upwards of 12% to 26% have been recorded in ACN fields(Doumbia et al., 2006). Soil organic carbon increases led to increased fertilizer efficiency, waterholding capacity, and permeability of the sandy soils found in much of Mali (Doumbia et al.,2006).<br><br>The main environmental service from ACN that was investigated in this study was the potentialincrease in recharge of local groundwater sources and reduction surface runoff. There has beenmuch anecdotal evidence from discussions with farmers about these impacts on watermanagement due to ACN (Gigou et al., 1999). This anecdotal evidence needs to be verifiedthrough assessment of field water balances.<br><br>Traditional runoff plot studies were not possible given costs and lack of institutional capabilitiesof the Institut dEconomie Rurale (IER) of Mali; therefore, field water balance calculations usingsoil moisture measurements were used to investigate the effect of the ACN on runoff and deeppercolation occurring in agricultural fields. Two treatments were applied near Siguidolo, Maliincluding ACN and a control. A capacitance based soil moisture probe was used to record soilmoisture vertically from 10 cm to 160 cm in two replications of 6 tubes per plot. Readings were taken by LInstitut dEconomie Rurale (IER) of Mali technicians on a daily basis during thegrowing season from June to September and once a month during the dry season. The soilmoisture observations collected were used in addition to readily available meteorological data toconduct water balance calculations for fields with and without ACN.<br><br>The water balance calculations considered all the major hydrologic processes at the field scale.The procedures used to estimate the field water balance in this study were based on theprocedures used by Paramasivam et al. (2001). Precipitation and soil moisture were measureddirectly. Soil evaporation and crop ET (Allen et al., 1998) were estimated using potentialevaporation calculations (Thornthwaite, 1948). Soil water storage was estimated from the soilmoisture measurements in the root zone. Deep percolation was calculated using soil moisturemeasurements below the root zone, pedo-transfer functions (Young et al., 1998; Vereecken etal., 1989) and Darcys equation (Van Genutchen, 1980). Finally, runoff was calculated using amass balance equation that considers each of the processes influencing the field water balance.The relative differences between the treatment and control for deep percolation and runoff wereused rather than the magnitude of these values. There is much uncertainty in the magnitudes.However, the accuracy of the relative difference was considered sufficient for comparisonbecause the errors resulting from the calculations would cancel out when the relative error wascalculated.<br><br>The field water balance calculations quantified positive impacts of ACN on deep percolation andrunoff. The ACN fields generally had greater deep percolation and lower runoff than the fieldswithout ACN (Figure 2). Generally, the ACN field tended to percolate more water than thecontrol during each rainy season. There were large precipitation events in June-July 2003. Forthese events, more precipitation was converted to deep percolation for the ACN fields than thecontrol. For the entire monitoring period, there was 132% more deep percolation from fields withACN than fields without. Overall, there were large reductions in runoff from the fields in ACNcompared to the control. For both replications, there was less runoff produced from the ACNfields than the control (Figure 2). As with deep percolation, the large precipitation events thatoccurred in June-July 2003 produced less runoff from the ACN fields compared to the control.The fields with ACN produced 94% less runoff than the fields using conventional farmingpractices (Figure 2).<br><br>Increasing deep percolation on agricultural fields is important for both local water resourcemanagement and larger concerns like cash crop production and revenue increases. Theincrease in deep percolation will also increase the recharge of local groundwater resources,which villagers rely on through wells as their sole drinking water source. Also, reductions insurface runoff and the re-direction of that water into the ground will reduce the potentialdisturbance and loss of soil, hence increasing the potential of carbon sequestration andimproving soil fertility. Finally, through this investigation the institutional capacity of IER in Maliwas increased by training and involving technicians in the collection of the soil moisture dataand scientists in the calculation of the field water balance, which is essential for the continuationand improvement of water resource management in Mali.