Temporal trends of hydro-climatic variables and their relevance in water resource management

Identification of temporal changes in hydrological regimes of river basins is an important topic in contemporary hydrology because of the potential impacts of climate change on river flow regimes. For this purpose, long-term historical records of rainfall (P), runoff (Q) and other climatic factors were used to investigate hydrological variability and trends in the Tajan River Basin over the period 1969–1998. Actual evaporation (E), rainfall variability index (d), evaporation ratio (CE) and runoff ratio (CQ) were estimated from the available hydroclimatological records. Mann-Kendall trend analysis and non-parametric Sen's slope estimates were performed on the respective time series variables to detect monotonic trend direction and magnitude of change over time. Rainfall variability index showed that 1973 was the wettest year (δ = +2.039) while 1985 was the driest (δ = -1.584). Also, decades 69–78 and 89–98 were recognized as the wettest and driest decades respectively. The gradient of variation of climatological parameters showed that during the study period, all three parameters of rainfall, evaporation and runoff have decreased and the variations of rainfall and evaporation were significant at the 95% level. Investigation of hydrological changes due of dam construction (1999) showed that the amount and annual distribution of discharge were completely different pre and post-dam construction. Discharge decreased in high water months and increased in low water months to meet water supply demands, especially for agriculture. The relationship between temperature and rainfall trends is compared for three stations in Mazandaran Province (Gorgan, Babolsar and Ramsar) from 1956 to 2003 and nine other stations with different statistical periods of 19–36 years, relating trends to northern hemisphere and global trends. Decreases in temperature were accompanied by decreases in rainfall, and vice versa. These trends were not observed in northern hemisphere and world scales, where temperature increases are accompanied by decreases in rainfall. These variations of hydroclimatological parameters show undesirable water resources situations during the statistical periods if the trend continues severe water resource crises.