Impact of contour soil bunds on soil moisture conservation and crop yield in the Bokole watershed, southwestern Ethiopia

Abstract

In Ethiopia, contour bunds are the main soil and water conservation measure. Their beneficial action can be attributed to a reduction in slope length and slope angle, which reduces the runoff velocity and the consequent soil loss, and an increase in infiltration. In turn, these benefits can positively influence crop yields as attested in earlier studies (Vancampenhout et al., 2006; Nyssen et al., 2007; Teshome et al., 2013; Wolka et al., 2016). However, these studies did not address the effect of soil bunds on the soil moisture content. Therefore, the present study is conducted to quantify the importance of contour soil bunds on soil moisture and yield and to indicate the effect of possible other parameters on yield, like rainfall, temperature, soil depth, slope and soil fertility. The study is conducted in two experimental fields cultivated with maize in the Bokole watershed in southwestern Ethiopia, which contain plots with and without soil bunds.

The FAO AquaCrop model is used to simulate the daily soil water balance, the crop development and the total attainable crop yield. The AquaCrop input data is determined by conducting laboratory and field measurements and using recommended values by the FAO (2012). The model was calibrated for the measured soil moisture content and the total aboveground dried biomass to get accurate simulations for the study area. After calibration, the FAO AquaCrop model simulates the soil moisture and the biomass well with regard to the measured parameters. To determine the effect of the soil bunds, the results of the plots with soil bunds are compared to the results of the plots without soil bunds.

There is no difference in soil moisture content between the plots with soil bunds and the plots without soil bunds. However, the variation of soil moisture within the plots result in small changes in maize yield, but no consistent pattern could be recognized. Furthermore, the biomass and yield are, on average, 11.6% and 13.6% higher in plots with soil bunds in field 1, respectively, but in field 2, the biomass and the yield are, on average, 21.0% and 21.5% lower in plots with soil bunds, respectively. This contradiction in the findings is mostly due to the difference in soil fertility stress, which is 5.1% lower for plots with soil bunds in field 1, but is 7.5% higher for plots with soil bunds in field 2. The soil fertility stress ranges between 25% and 80% between the plots and is the dominant parameters leading to the variation in maize yield between the plots. The maize yield differs from 1.5 ton ha-1 to 5.7 ton ha-1 in field 1 and from 1.3 ton ha-1 to 2.6 ton ha-1 in field 2. When the variation in soil fertility stress is eliminated it can be seen that the maximum green canopy cover, the soil depth and the gap between the soil moisture content at field capacity and at saturation are also important parameters affecting the maize yield. A higher maximum green canopy cover and a deeper soil positively influence the yield, while the gap between the soil moisture content at field capacity and at saturation only significantly influence the yield when it is approximately smaller than 5 %vol, which triggers aeration stress more easily. In addition, when comparing the maize yields produced under different weather conditions, which are based on the weather conditions from 2005 to 2016 in the Bokole watershed, it is clear that the temperature has more influence on the maize yield than rainfall in the Bokole watershed. On the whole, no significant differences in parameters between plots with and without soil bunds are observed. This is most likely due to the young age of the soil bunds and the sufficient availability of water, which makes the immediate effect of soil bunds with regard to soil conservation unnecessary. Nevertheless, the maize yield varies between the plots, which is observed to be dominantly caused by variation in soil fertility stress, maximum green canopy cover, soil depth and slope between the plots. Moreover, the soil moisture at permanent wilting point, at field capacity and at saturation influence the water and aeration stress and, therefore, variation in these soil properties cause small differences in the maize yield.