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amount of groundwater was withdrawn from the study area over the study time, which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,
