| dc.description.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. | |
