Assessment and evaluation of hydraulic parameters in Raya irrigation pumping wells

Abstract

The Raya well field located in the northern part of Ethiopia, southern part of Tigray region and have raised concerns with respect to the ability of the aquifer system to sustain production. As the aquifer of Raya well field in particular is heavily used in this area, understanding the hydraulic parameters of the area is essential for efficient sustainable management of the groundwater resource. An overview of the hydraulic parameters estimated for the 22 pumping irrigation wells in the study clearly show strong evidence of inappropriate estimation. Thus, this study was conducted to identify the possible miss-interpretation of the observed time-drawdown, step drawdown, time residual drawdown-based hydraulic parameter estimation. After data quality checking, appropriate data correction for partial penetration, well loss and unconfined aquifer were conducted. Then the standard methods like Theis curve fitting, Cooper-Jacob methods for time drawdown data & Theis recovery method for time-residual drawdown data were used for parameter estimation. Beside Aquifertest v8.0 software was also used to verify the results of the original study. Evaluation and analysis result showed that out of the 22 well analyzed, 15 transmissivity values used in the well yield estimate were overestimated (0.22 up to 29.23 percent) and 6 underestimated (0.26 up to 1700.72 percent) the other one well has not stated transmissivity value, such highly overestimated transmissivity value misleads the designers & operators in fixing the well yield, 15-25% cost overrun, Poor water supply reliability, planning inequity and trust erosion with the client or society and reduce the life span of the pumping wells by 26-62 percent. Additionally, from 22 pumping wells 18 or 81.81 percent out of the total sample wells have not efficiency stated in the design documents. The minimum efficiency of irrigation wells should be 60%, considering this minimum irrigation wells efficiency limit 3 pumping wells have efficiency greater than 60 percent this is around 13.63 percent the others 19 pumping wells have less than 60 percent efficiency, this is around 86.36 percent. The storativity of unconfined aquifer is 0.1-0.3 and the storativity of confined aquifer is 0.005-0.00005. data from Tigray office of water resources and energy shown that the storativity of the 13 wells are far away from the range whereas 2 wells are slightly out of the range. The rest 7 wells have not storativity stated in the document. results from this research computations shown that the storativity of the 14 wells are far away from the range, whereas 7 wells are slightly out of the range. The rest 1 well falls within the range or accepted. the reason behind this large error in storativity value is the wells are only pumping or production wells, for accurate and precise storativity computation an observation well is very mandatory. The recommendation for the less efficiency is to Reduce the pumping rate or Optimize Pumping Operation. Results also shown that most of the irrigation pumping wells fall on the category of class of transmissivity magnitude “II” and fall on the “high” designation of transmissivity magnitude. a well pumped for 18 hours per day and for 120 days in a year it can lead to cumulative land subsidence from 0.009 meter or 9 mm up to 0.056 or 56 mm per year. Therefore, in this evaluation it is recommended to conduct multiple well test to determine the actual hydraulic parameters of the aquifer in order to obtain the safe yield of the wells which is intended to prolong the productivity of the aquifer and to delay excessive loss of saturated thickness.