Geographical and hydrogeological settings in defining threshold spacing of shallow hand-dug wells : a case study in Debrekidan Watershed, North Ethiopia

Abstract

The success of irrigation agriculture in many areas of north Ethiopia is depend on the availability of shallow ground water and utilization of shallow hand-dug well for irrigation purpose. However, intensive construction of shallow hand-dug wells resulted in the problem of well dry-up and irrigation water shortage. Taking Debrekidan watershed as an example, this paper describes the geographical and hydrogeological settings of the watershed, and finally suggests the threshold spacing of hand-dug wells for rational utilization of groundwater resource. To evaluate possible causes of irrigation water shortage, the assessment was carried on well spacing, type and nature of aquifer, upper position of ground water level, geographical position of wells, the dimensions of wells, recharge and discharge rate of wells, frequency of watering practice, and availability of irrigation water by adopting systematic sampling method. Based on the data collected from representative wells sample, spacing and well depth have strong relationship with the problem of water shortage. Graphical presentation of spacing-availability of water shortage show that the probability to get wells with water shortage increases as much as 100% at a spacing less than 10m. At a spacing of greater than 50m, the graph predicts 15%. Regarding to well depth, the probability of getting wells with water shortage reduces as much as 10% at a depth of 8.5m. Based on the current well spacing-irrigation water availability relationship, the recommended spacing from the nearest well with minimum risk of water shortage is greater than 35m. The actual extents that narrow spacing cause’s water shortage and well dry-up, however, can not be fully predicted on the base of data analyzed because many other factors may adversely affecting the recovery of the wells.