تغيير پذيري منابع آب در حوضه آبريز سد لتيان و اثر تغيير اقليم بر آن

Water Resources Variability in Latian Basin and the Effect of Climate Change

Introduction The aim of this study is to clarify the impact of climate change on water resources variability. To achieve this, the relationship between the discharge, temperature, and precipitation as well as their trend is investigated using statistical procedures. Methodology First the trend of discharge, temperature, and precipitation records is detected using the Kendall-tau test (Robson et aI., 2000). Kendall-tau test is a rankbased test which determines whether the null hypothesis is rejected at significance level a if: Izl>ZI_iJ (I) where z(a/ is the value of the standard normal - /] distribution with the exceedancc probability of iJ . In the next step, a few models are generated for the annual mean discharge and the daily mean discharge in each month using linear regression. Sinee the monthly distribution of precipitation is an important factor affecting the annual mean discharge, the monthly precipitation and the temperature were used as independent variables for the annual discharge modeling. For the daily discharge modeling, the daily precipitation and the temperature of the same month, the total precipitation and the mean temperature of the previous month were used as independent variables For the model validation, a few models were generated using the first 90 percent of the available records. Then the last 10 percent of the discharge time series were estimated using these models and the correlation coefficient between the measured and the estimated values were determined. Results The results show that there is no significant trend for the annual mean discharge time series during the past four decades (1341-1381). But the amount of monthly discharge has increased in the cold season (Azar, Dey. Bahman, Farvardin) and decreased in the wann seasons (Khordad, Tir, Mordad, Shahrivar), Table I shows the Kendall-tau test coefficient and the rate of discharge changes in each month. during the past four decades. The Kendall-tau test showed no significant trends for the annual and the monthly precipitation records during 1346-1381. Based on the Kendall-tau test results, both the annual and the monthly temperature records (except for Esfand) has increased during the past four decades (1341-1381). Table I shows the generated models for the annual and the daily discharges in each month, correlation coefficients between the measured and the estimated records and also the correlation coefficients for model validation (X1 to X 12 represent precipitation of Mehr (October) to Shahrivar (September) and X j~ʹ to Xc; represent temperature of Mehr (October) to Shahrivar (September), respectively.. According to these models the amount of the total precipitation of Aban, Azar, Bahman, Esfand, and Ordibehesht and also the mean temperature of Farvardin signify the amount of the annual discharge. The most important variable in the fluctuation of the daily mean discharge of Mehr and Aban is the precipitation of the same day. This is because the temperature is high and the form of precipitation is in liquid and therefore it transforms into discharge immediately. After Azar due to the reduction of temperature and the delay in discharge reaction to precipitation, the role of the daily precipitation decreases gradually till Esfand. Accordingly the effect of the daily temperature on discharge increases. Therefore the daily temperature and the monthly total precipitation of previous months are the important variables in determining discharge in these months. After Ordibehesht, the temperature affects the discharge inversely and therefore, the monthly temperature and the total precipitation of previous months are the main variables to determine discharge. Conclusion It can be concluded that the discharge of the rivers in Latian basin has increased in cold seasons and decreased in warm seasons as a consequenee of the temperature increase. The reason is that the main portion of the precipitation in the eold season oeeurs as snow. Therefore, it may be inferred that this area will suffer a substantial decrease in river discharge in warm seasons.