The water of several perennial streams in the Messica catchment in Mozambique is increasingly used by farmers for irrigation of their small-scale furrow irrigation systems. Eight months of daily discharge measurements were available for Rio Godi, which is one of the perennial streams. With these measurements the HBV light model was calibrated and subsequently the model was used to investigate the effect of climate change on future water availability during the dry season.
Climate change impact on the water availability in the Messica catchment in Manica Province, Mozambique - A case study of Rio Godi
The Messica catchment is situated at the western border of central Mozambique in the Manica province and covers an area of approximately 220 km2. The catchment is drained by the Rio Messica, which is fed by several perennial streams. More and more farmers make use of the opportunity to divert water from the small streams to their land and irrigate their crops during the dry season. Although in general the planted area per farmer is small, crops are not only grown for subsistence farming, but also for commercial purposes. Therefore the development of small-scale irrigation systems can contribute to the economic development of the region.
In the past few years the development of informal small-scale furrow irrigation systems was observed and a further expansion of irrigated area is expected in the coming years. However, there is a limit to the total area that can be irrigated during the different cropping seasons. Although water use efficiency plays a role, the limit depends primarily on water availability during the dry season. Therefore in this study the long-term water availability of Rio Godi (10.13 km2), one of the streams feeding the Rio Messica, was investigated. Daily discharge was derived from automated water level measurements that were recorded between December 2012 and August 2013. Since the discharge of Rio Godi was influenced by irrigation, the measured discharge at the catchment outlet was naturalized before the data was used for calibrating the lumped conceptual rainfall-runoff model HBV. Predictions for future water availability during the dry season were made for the period 2021-2050 and the period 2071-2100 by running the calibrated model with the down-scaled and bias-corrected output of the Global Circulation Model CNCM3 (high A2 emission scenario). These data were obtained from the EU-FP6 WATCH (WATer and global Change) project. Comparison with the reference run that was done with WATCH Forcing Data for the period 1971-2000 showed that the probability distribution of the total water availability of the dry season hardly changed. However, the minimum flow during the dry season shows a slightly decreasing trend for half of the years for the period 2071-2100. Still, the influence of climate change on water availability will probably be minor compared to the influence of the development of the small-scale irrigation. The challenge of ensuring water availability for all water users during the dry season in future must therefore be focussed more on optimizing the water use efficiency, minimising the water losses, and balancing between water availability and further development of small-scale irrigation.