Analysis of Spatiotemporal Changes in the Baseflow of Iran’s Rivers over the Past 30 Years

The aim of this study is to estimate and analyze the spatiotemporal changes in the baseflow of 266 rivers across Iran in a 30-year period (1987-2017) in order to determine the degree of influence of groundwater sources and snowmelt in the studied rivers. The daily baseflows were separated from the streamflows recorded at the hydrometric stations using the Chapman-Maxwell digital filter method. A non-parametric Mann-Kendall test was used to analyze the baseflow time trend, and Moran's I spatial autocorrelation index was used to analyze the spatial autocorrelation of baseflow and baseflow index (baseflow ratio to streamflow). The monthly analysis of baseflow showed a regular seasonal pattern with the highest and lowest values of 9.08 and 1.95 million cubic meters per month, corresponding to the months of April and September, respectively. The results of the baseflow index showed that the share of baseflow in the surface water supply of the studied rivers is between 0.15% and 0.99% (72% on average). The results of the long-term trend of the baseflow indicated that 83.08% of the rivers experienced a significant downward trend (at a level of 0.95) in the studied period. The findings of the spatial autocorrelation test confirmed that there are several clusters with high baseflow and baseflow indexes in the Zagros and Alborz mountain ranges. The results of this research can provide a general picture of the temporal and spatial changes in the baseflow of rivers at the scale of the country and provide outstanding help to decision-makers in order to achieve integrated management of surface water resources.