Statistical Analysis of Long-Term Changes in Precipitation, Discharge, and Land Use (Case Study: Tajan Watershed, Northern Iran)

Water stresses are one of the most important issues that the world is facing today. Following the increase in population, climate, and land use change, Iran is also facing serious challenges in achieving sustainable water management on basin scale. In recent decades, climate and land use changes at the level of watersheds have had adverse effects on socio-environmental and economic issues. The case considered in this study is the Tajan watershed, located in Mazandaran province, northern Iran. This watershed is crucial in terms of ecosystem services (runoff generation, sediment delivery) for the residents living in the area and the Caspian Sea, which consists of the terminal part of the watershed. Temporal analysis of trends in hydro-climatic data is important to increasing our knowledge about the variations of such variables over time.

Tajan watershed with an area of 3,970 square kilometers is located in the northern part of Iran, which discharges the generated runoff into the Caspian Sea. Contrary to other watersheds, the amount of rainfall decreases with increasing altitude (i.e., the gradient of precipitation is negative with the change in altitude). An analysis of the temporal trend of the precipitation and discharge was conducted using the long-term monthly data (50 years from 1971 to 2021) collected in four rain gauges and two hydrometric stations. The rescaled adjusted partial sums (RAPS) index, non-parametric Mann-Kendall test, and Sen’s slope method were adopted to reveal the changes in precipitation and discharge data. Moreover, the state of land use changes in 37 years (1984-2021) were extracted using the Maximum Likelihood Classification method on the satellite images.

The results of this study showed that the trend of discharge does not match the rainfall, because during the 30 years of 1969-1999, the rainfall data of the studied stations had a decreasing trend with an average slope of -0.463 mm/year, while the discharge in watershed outlet indicates an increasing trend with an average slope of +1.02 cubic meters per second per year. On the other hand, during the period between 1999-2021, despite the positive trend of precipitation data with an average slope of +0.343 mm per year, the discharge shows a decreasing trend with an average slope of -2.249 cubic meters per second per year. By examining the state of land use changes in 1984 and 2021, the results indicate a decrease of 16 and 11 percent of areas for pasture and forestlands and an increase of 110 and 670 percent of irrigated lands and urban areas. The findings of this research showed that the trend of temporal changes in the discharge of sub-basins does not follow the rainfall, and the main factor in reducing the runoff generated in the Tajan watershed is the land use changes over the studied catchment.

The following conclusion could be drawn upon an evaluation of the temporal changes in rainfall and runoff data in the Tajan watershed: 1) the change of land use in the study area from forest to agricultural and urban areas caused an increase in the volume of runoff generated in the watershed. 2) The construction of the Shahid Rajaei reservoir dam in the study area has caused a change in the discharge trend of the mainstream. This affects the region's ecosystem as well as the morphology of the river channel that passes through the city of Sari. 3) The hydrological response of the studied watershed is not only influenced by climatic factors, the human activities (e.g., land use change and damming) has significant contribution. The authors suggest future researchers conduct more extensive research on the effect of dams on the hydromorphological characteristics of the downstream river.