Streamflow Response to Rainfall Changes Using the Climate Elasticity Index in Some Watersheds of Ardabil Province

Determining the sensitivity of streamflow to climate is necessary to make informed decisions to manage water resources and environmental systems for predicting hydro-climatic variability and climate change. Climate variability is considered as a key driver of hydrological processes. The sensitivity of streamflow to climate variables is predicted using a hydrological modeling procedure. In this regard, the results of streamflow modeling are comparing through the present and projected climate scenarios. Climate change in the last century has largely affected the processes of the water cycle and its components on different spatial scales. In recent years, the identification of effective factors and their impact on regional runoff changes has been widely explored by researchers in the field of hydrology. In the context of exploring water resources due to climate change, it is easy to estimate the impact of climate on political decision making and planning. Precipitation elasticity is defined as a tool to determine the rate of streamflow sensitivity regarding the precipitation variability. This study aims to calculate rainfall elasticity and variation of discharge in 20 watersheds using nonparametric elasticity estimation in the monthly timescale.

In this study, the sensitivity of rainfall to precipitation has been calculated using nonparametric estimation and a set of monthly data and precipitation data for Ardabil province. Climatic elasticity can be calculated by dividing the climatic variables such as rainfall, relative humidity, temperature, evapotranspiration, wind speed, specific radiation, etc. To estimate the Elasticity of precipitation (Ep), a non-parametric estimation of a set of average monthly discharge and rainfall data is required. At first, the monthly precipitation elasticity was calculated for 20 river gauge stations in the study area, and the median of these values was estimated as precipitation elasticity for the entire province in 12 months of the year. Then the Triple Diagram Model was used to assess the changes in the precipitation elasticity index with precipitation and discharge values. Also, based on the range of changes in the elasticity index, the hydrometric stations studied were classified into 3 categories and presented through a spatial map.

The results showed that the range of the elasticity index was between -2.21 to 3.96, which is related to Arbabkandi and Shamsabad stations, respectively. Based on the results of the Triple diagram model, the variability of the elasticity index is higher in the low discharges. Also, the value of the elasticity index is higher in the dry months, than the other months, which proves the greater impacts of precipitation on the river flow rising in dry months. There is also an inverse relationship between the elasticity index and the upland watershed area of each river gauge station. In watersheds located in upland parts of the area, the discharge shows fewer changes than precipitation, while in downstream watersheds, the discharge is changing more with precipitation variations. According to had the monthly elasticity-precipitation diagram, the calculated elasticity values had a higher amount in the range of medium values of precipitation (0-20 mm and 15-15 mm) in dry and wet months, respectively.

 Discussion and conclusion

The results showed that the sensitivity of the elasticity index is higher at low discharge values, while in the higher values of the discharge, the elasticity index is less sensitive. According to the results, in the dry months, the value of elasticity index is higher than other months; in this case, it is possible to refer to the sensitivity of the change in rainfall to dry rainfall during the dry months. Changes in the values of the elasticity index in different rainfall indicate that the value of the low elasticity index was attributed to the precipitation occurs in the cold months of the year as a snowfall, which related to the delayed response of snow melting. In particular, due to snowmelt in upstream watersheds, this time delay reduces the elasticity index. It is also very difficult to distinguish the effects of human activities and changes using the employed approach. On the other hand, the sensitivity of the river flow varies over the study area, and it is always different considering the changes of climatic components, human exploitation, land use, geological characteristics, etc. In particular, calculating the elasticity index allows comparing the behavior of different rivers in terms of response to climate change changes.