A structured framework to interpret hydro-climatic and water quality trends in Mediterranean climate zones

Abstract

Many regions of the world have experienced rapid hydro-climatic changes in recent decades. Mediterranean areas have especially experienced prolonged droughts, changes to runoff, and low-flow periods. Many methods and assumptions for assessing trends in rainfall and runoff are available, however, a comprehensive framework is required that can consider variations at large temporal and spatial scales, long-term persistence (LTP), seasonal patterns, and multiple change points. An analysis framework has been developed in R and applied to a network of 107 rainfall gauges (with about 100 yrs data) and 90 streamflow-water quality gauges (30 – 50 yrs data) across South-west Western Australia, a region with a typical Mediterranean climate. Trends were estimated using the Original Mann–Kendall (OMK) family of tests including using the Mann-Kendall Test under the Scaling Hypothesis (MKLTP) and Seasonal Mann–Kendall (SMK). Single and multiple change points were calculated by the non-parametric Lanzante’s test (LAT), E-Agglomerative algorithm (ECP), and Energy Divisive (EDP). Trend tests show annual rainfall has decreased by 2.6 mm/yr in the Western areas and increased by 0.9 mm/yr in the Eastern areas with corresponding change points in the 1940 s-1960 s and from 1995 to 2000 although 30 % of the annual rainfall and 100 % of the standardized precipitation index (SPI) time series were found to be under LTP. A regional flow trend analysis shows catchments are yielding much less runoff in more than 90 % of gauged stations. The runoff in perennial and ephemeral streams has shown a decline with the rate of 6 (54 %) and 2 (50 %) mm/yr, respectively. Runoff and baseflow decline occurred as step changes lagged after rainfall, and the rate of decline in baseflow was more than for runoff, suggesting that groundwater connectivity with stream inverts has fallen substantially. Stream salinities increased by 64 mg/L/yr (or 15 %) overall which was highly significant. The framework provides a structured approach to analyse changes in rainfall, runoff and stream salinity for a Mediterranean climate that is projected to become even warmer and drier.