| Title: | Water availability, demand and reliability of in situ water harvesting in smallholder rain-fed agriculture in the Thukela River Basin, South Africa |
| Authors: | Andersson, J.C.M., A.J.B. Zehnder, G.P.W. Jewitt and H.Yang |
| Year: | 2009 |
| Journal: | Hydrology and Earth System Sciences |
| Volume (Issue): | 13(12) |
| Pages: | 2329-2347 |
| Article ID: | |
| DOI: | 10.5194/hess-13-2329-2009 |
| URL (non-DOI journals): | |
| Model: | SWAT |
| Broad Application Category: | hydrologic only |
| Primary Application Category: | rainwater/water harvesting or utilization |
| Secondary Application Category: | crop, forest and/or vegetation growth/yield and/or parameters |
| Watershed Description: | 29,000 km^2 Thukela River, which drains to the Indian Ocean in southeast South Africa. |
| Calibration Summary: | |
| Validation Summary: | |
| General Comments: | |
| Abstract: | Water productivity in smallholder rain-fed agriculture
is of key interest for improved food and livelihood
security. A frequently advocated approach to enhance water
productivity is to adopt water harvesting and conservation
technologies (WH). This study estimates water availability
for potential in situ WH, and supplemental water demand
(SWD) in smallholder agriculture in South Africa’s
Thukela River Basin (29 000 km^2, mean annual precipitation
550–2000mmyr−1). The study includes process dynamics
governing runoff generation and crop water demands, quantification
of prediction uncertainty, and an analysis of the reliability
of in situ WH. The agro-hydrological model SWAT (Soil and Water Assessment
Tool) was calibrated and evaluated with the Sequential
Uncertainty Fitting algorithm against observed discharge
(at ten stations) and maize yield (the dominant crop
type) for the period 1997–2006. The water availability was
based on the generated surface runoff in smallholder areas.
The SWD was derived from a scenario where crop water
deficits were met from an unlimited external water source.
The reliability was calculated as the percentage of years in
which water availability >=SWD. This reflects the risks of
failure induced by the temporal variability in the water availability
and the SWD.
The calibration reduced the predictive uncertainty and resulted
in a satisfactory model performance. For smallholder
maize yield, the Root Mean Squared Error was 0.02 t ha−1
during both the calibration and the evaluation periods. The
width of the uncertainty band was reduced by 23% due to
the calibration. For discharge during the calibration (evaluation)
period, the ten-station range in the weighted coefficient
of determination (8) was 0.16–0.85 (0.18–0.73), and in the
coefficient of determination (R^2) 0.42–0.83 (0.28–0.72). The
calibration reduced the width of the uncertainty band by 25%
on average.
The results show that the smallholder crop water productivity
is currently low in the basin (spatiotemporal median:
0.08–0.22 kgm−3, 95% prediction uncertainty band
(95PPU)). Water is available for in situ WH (spatiotemporal
median: 0–17mm year−1, 95PPU) which may aid in enhancing
the crop water productivity by meeting some of the
SWD (spatiotemporal median: 0–113mm year−1, 95PPU).
However, the reliability of in situ WH is highly location specific
and overall rather low. Of the 1850 km^2 of smallholder
lands, 20–28% display a reliability >=25%, 13–16% a reliability
>=50%, and 4–5% a reliability >=75% (95PPU). This
suggests that the risk of failure of in situ WH is relatively
high in many areas of the basin. |
| Language: | English |
| Keywords: | |