Statistical analysis of the SPI index and discharge potential of the Khansar Sarcheshmeh springs

Planning the cultivation pattern and water rights allocation in the irrigated lands by the Khansar Sarcheshmeh springs requires knowledge of the discharge of the springs, estimated based on the Standard Precipitation Index (SPI) in this research. The relationship between the SPI index, precipitation, and discharges of Khansar Sarchesmeh springs has been evaluated over three different periods to estimate the groundwater resources and inflow to the streams. In a year with normal or near-normal climatic conditions and average annual rainfall between 295 to 494 mm, the average annual discharge of Sarcheshmeh springs is expected to be between 12.8 and 24.3 million cubic meters (average of 18.1 million cubic meters) with 67.5% probability and a frequency of 1.5 years based on the extreme value distribution. For a year with average dry climatic conditions (270 mm average annual rainfall), the predicted average annual discharge of Sarcheshmeh springs is 11.9 million cubic meters with a 9% probability and a frequency of 11 years. In extremely dry climatic years (with an occurrence frequency of 28 years), the discharge of Sarcheshmeh springs is expected to decrease to less than 10.1 million cubic meters per year. In normal and close to normal conditions, with average rainfall in the range of 236 to 326 mm in the first six months, the average annual (water year) discharge of Sarcheshmeh springs is expected to be 18.5 million cubic meters (ranging from 15.3 to 21.8 million cubic meters with a 67.5% probability based on the extreme value distribution). According to the results of this research, rainfall of less than 168 mm occurred in the first half of the water year is a warning of the occurrence of a moderate to extremely dry drought with an annual discharge of less than 11.6 million cubic meters that can be distributed in the streams. Precipitations of more than 394 mm during the first half of the water year also indicate the likelihood of a moderate to extremely wet year, with an annual rainfall of more than 28 million cubic meters. Results of the research indicate that less than 168 mm of rainfall in the first half of the water year signals the potential for a moderate to extremely dry drought, with an annual discharge of less than 11.6 million cubic meters that can be distributed in the streams, while more than 394 mm of precipitation during the same period suggests the possibility of a moderate to extremely wet year with an annual rainfall of more than 28 million cubic meters.