Impact of groundwater irrigation on dry season flow of the Tangan river

Abstract

In Bnngbde8b groundwater irngn1Jo[] " a common I'rnclLcc lor agricultural pro<!uctlOll IncreJsed lI>;CoC groundw<ller m'igation for ngricullural uses mny ha\'e resulted in dcclincs of dry season stle~mflow and hasdlow, In thl8 study, an attcmpt was mude to inve5\lgate the rciatlOn of thc Tangan River dry ,cason flow amI bascllow with the ahstractlOn of groundwatcr m il~ catchmcnt through the systematic use of ;tatistical and basel10w separation techniques, Dala analyzed include ~treamf1ow, baseflow, and groundwatcr I~vel,. The b"sel1()w separation WilSdOllc using the sofhvarc BFLOW, Rom ,cnson (J,lllUilryto early May) W<lS~on~idered for the study smee groundwater is the only source of irrigotion during thcse four months. Average deeadal (10- day) streamflow and b~senow were calculiited I"orthcse four monlhs over a pcriod from 1973 to 19<)3, Time series of average deeadal srlreamflow and basellow showed a declining trend of nver now, The study estimated the percentage of avcrage streamflow and haseJlow reduction in the Tangall River during these four months period over the study time of 1973 to 1993 and these ,vere founl! to be 41 % and 35 % respectively. Trend line filted to the lowest groundwater table wa;; also found to be downward and this indIcates the lowering of groundwatcr table through the time pcriod of 1973 to 1993. An increased amount of groundwater was withdrawn from the study area over the study time, which wa, estimated from dwwdo''''1 alld specific yield data. A correlation coefficient of -0,66 bet,veell (he strcamllow and groundwater abstraction was obtilined for the period from 1973 to I ~93. For basellow and groundw"ter ilb,tmc\ion, the eorrel~tion c()efficient was fouml to be -0,67, l'urlhemlOre, for baseflow, the C()lTelation cocf1ieient increases to -0.91 if data from 1978 are considered due to their better reliability. Groundwater abstrilclH)n correlates well With baseflow (-0.91) than streamflow (-0,66). With I unit incrcase of groundwater abstraction, baseflow of Ihe river would decrease by about 0.2 uml. The statistical analYSISmdicates th~lthe primary factor causing the streamflow and baseflow declines i~ lowered gl'Oundwater level callscd by increased abstraction mainly fmnl dry season irrig,ltion,