Spatio-temporal variation of temperature characteristics over Narmada basin - is the consistent warming trend a possible climate change signal?

The analysis of the high resolution gridded data (1 degrees x 1 degrees) of daily maximum, minimum and mean temperature for the 23 grids covering the Narmada basin has detected varying trends in the upper, middle and lower zones of the basin, based on the 40 years data from 1961-2008. The annual 1- day maximum temperature has been steadily increasing at the rate of 1.10 degrees C/100 yr, but the annual 1- day minimum temperature depicted a much higher rate in increase of 3.20 degrees C/100 yr with significant rising trend at 95% significance level (test statistic: Z = +1.989) based on the non-parametric Mann-Kendall test for detection of trend in time series. The zone-wise temperature variation has been studied for two distinct time periods, each of 20 years viz. 1969-88 and 1989-08. A divergent pattern is observed in minimum daily temperature trends in the various zones of the basin with significant rising trends in the lower zone and no significant trend in the upper zone during both time periods. Also a significant rising trend has been detected in the diurnal temperature range (DTR) in the lower zone during August (Z = + 2.21), with no significant trends in other months. Significant rising trends have been detected in the mean monthly temperatures in the upper and middle zones, with no significant trend in lower zone. Similar pattern of significant rising trend have been detected in mean monthly temperatures during the principal winter (December and January) and summer months (April and May) in upper and middle zones of the basin. However, no significant trend has been detected during the principal monsoon months (July and August) for all the zones. As the basin is located in a semi-arid region and the perennial flows result from the contribution from the soil moisture and groundwater storages, any increase in the maximum and minimum temperatures will change the water availability scenario due to increased water demands from higher consumptive use requirements. To study the water demand scenario in the various zones in the basin, the potential evapotranspiration (PET) has been computed. Significant rising trends in PET were detected in all the zones, during the 1989-08 time slot, which indicates higher crop water requirement from the agriculture sector due to the increased temperatures. The comparison of the zonal annual rainfall (AR) with the actual evapotranspiration (AET) which gives an indication of the supply-demand scenario, has found that the annual AET has been greater than AR in the middle zone on 5 occasions during 1969-88 and subsequently increased to 10 occasions during 1989-08, whereas in the lower zone the AET was greater than AR during most of the years in both time periods. This indicates increased water demands and lower rainwater availability under the changing climate scenario, in the middle and lower zones of Narmada basin, which calls for effective water resources development and management strategies.