Analysis of return flows from the upper Mabogini irrigation scheme: the case of lower Moshi irrigation scheme in Moshi, Tanzania

A study was conducted in Town of Moshi, Tanzania in an irrigation scheme at Mabogini. The irrigation scheme draws its water from Rau River. The downstream part of the scheme runs completely dry in dry season due to excessive abstractions of water for different purposes mainly irrigation.There is water problem in the Rau Catchment due excessive water abstractions from the Rau River mainly for irrigation purposes .The study aimed at mapping and quantifying the shallow groundwater flow and to establish whether or not subsurface flows of water after irrigation find its way back to recharge the river. In this case piezometric wells were augured/dug and water levels in the piezometers were recorded. A total of 50 auger piezometer holes spaced at 60 m were drilled in an area of 135.25 ha. Using piezometric levels, hydraulic gradients were established and used to map groundwater flow. This enabled to predict the groundwater flow direction. A feasibility study conducted by the Japan International Cooperation Agency (JICA) showed that ground water table in the area can be found between 3 - 6 m. this indicated that groundwater can easily be reached and studied.Data analysis (Hydraulic head, land surface elevation, piezometric head coordinates) and interpretation was done using software spreadsheet, Surfer8 and GIS. From hydraulic head distribution and shallow ground water map, ground water seems to flow toward the Rau River in the area near the river banks. This happens near the river especially between 318600 X, 9625200Y and 318900X, 9625200Y. At the middle of the farm, shallow ground water moves toward recharging the aquifer at the rate of 0.84m3/d. Since much of the irrigation water seem to percolate to the aquifer rather than the Rau River, it is recommended that further abstractions for irrigation from the river be stopped.