GIS Supported Irrigable land and Water Resource Potential Investigation tor Irrigation (The Case of Hare River Watershed)

Abstract

Water resources development and utilization is usually not integrated with Ethiopia's overall socio-economic development objectives. In the above regard, one of the actions included in the strategy encourages to undertake assessment and development of the country's surface water resources by, among others, making inventory of the available surface water in the country both in quantity and quality and applying suitable methodologies to assess the available surface water both in time and space. As integrated development of water resources begin with an inventory of available surface water, this paper in particular is focusing on the investigation of availability of water for irrigation by implementing modern tools like the Geographic Information System (GIS) and Watershed Modeling Software (HEC-HMS) considering the Hare river watershed as representative of the study. With in the Hare watershed, four sub-basins (namely upper Harrere, Lower Harrere, Gina and Lower Hare) were identified for these study based on the threshold of their contribution to flow. The flow which will be generated from these sub-basins will be one set of decision parameter weather the area at the outlets of the sub-basins can benefit from the use of irrigation. From the result of the analysis of suitable irrigable land, Lower Hare sub basin contributes to most of the irrigable land (about 63% of sub-basin extent) followed by the Gina sub-basin (about 47% of sub-basin extent). The combined lower and upper Harrere sub-basins contribute much less (about 43% of combined sub-basin extent) as compared to the separate land contributed by the other two sub-basins. As a result of this irrigation demand assessment focuses primarily on crops cultivated on the Gina and Lower Hare Sub-basins. Assisted by the CROPWAT software, the requirements of the different crops in the respective sub-basins were made. It was observed that the Gina sub basin doesn't require any sort of diversion from the Gina River as the natural rainfall is sufficient to fulfill the crop water requirement while the scenario is quite the opposite for the lower Hare sub-basin. The runoff from Gina, Lower Hare and Upper/Lower Harrere was generated using the HEC-HMS package. The runoff generated from the upper two sub­ basins (i.e. Gina and Harrere) was routed through to the outlets at Lower Hare. It was observed that much of the contribution of the runoff at the lower Hare area comes from the Gina sub-basin. Finally, a comparison was made between the water supply from the main river and the irrigation water demand with in the adopted frame of reference for the Lower Hare sub basin where water stress is obvious. There was a deficit zone observed in the period of analysis. This deficit zone during the critical crop growth stage represents more than 55% of the total time of analysis which means that for more than half of the crop duration, the crops are subjected to water stress. This result was helpful to conclude that this shortage needs to be compensated by means of irrigation applications. Also with in the year though twice harvesting was possible, there are moments observed where water stress exists and need to be compensated if desired crop production level is not meant to reduce.