Effectability of SPI-based Watershed Health Index from Data Length

Watershed health provides an integrated view of the factors influencing the watershed conditions. In the current study, the data length effect on changes in SPI-based watershed health index was evaluated using the RelResVul framework for the period of 1961-2021 in Sahneh, Kermanshah, and Harsin stations in Kermanshah Province, Iran. For this purpose, reliability (Rel), resilience (Res), vulnerability (Vul) indicators, and mean watershed health (RelResVul) were calculated in different periods. According to the results, watershed health indicators were different in 1961-1980, 1981-2000, and 2001-2021. In addition, with the increase in the length of the statistical period, health indicators had different changes, so that in short-term data, the changes of the indicators mentioned above were more than the whole period. The maximum changes in the first 30 years of the statistical period, and with the increase in the length of the statistical period until the end of the period, changes decreased. Accordingly, the last 30 years of the statistical period have reliable results in calculating health indicators. Considering massive calculations, climatic data deficits, and limited stations while increasing the length of the statistical period, it is suggested to use the climatic data of the last 30 years to estimate watershed health.