Abstract
Abstract. Although water balance components at the catchment scale are strongly related to annual rainfall, the availability of water resources in Mediterranean catchments also depends on rainfall seasonality. Observed seasonal anomalies in historical records are fairly episodic, but an increase in their frequency might exacerbate water deficit or water excess if the rainy season shortens or extends its duration, e.g., due to climate change. This study evaluates the sensitivity of water yield, evapotranspiration, and groundwater recharge to changes in rainfall seasonality by using the Soil Water Assessment Tool (SWAT) model applied to the upper Alento River catchment (UARC) in southern Italy, where a long time series of daily rainfall is available from 1920 to 2018. We compare two distinct approaches: (i) a “static” approach, where three seasonal features (namely rainy, dry, and transition fixed-duration 4-month seasons) are identified through the standardized precipitation index (SPI) and (ii) a “dynamic” approach based on a stochastic framework, where the duration of two seasons (rainy and dry seasons) varies from year to year according to a probability distribution. Seasonal anomalies occur when the transition season is replaced by the rainy or dry season in the first approach and when season duration occurs in the tails of its normal distribution in the second approach. Results are presented within a probabilistic framework. We also show that the Budyko curve is sensitive to the rainfall seasonality regime in UARC by questioning the implicit assumption of a temporal steady state between annual average dryness and the evaporative index. Although the duration of the rainy season does not exert a major control on water balance, we were able to identify season-dependent regression equations linking water yield to the dryness index in the rainy season.
Highlights
The rainfall regime of the Mediterranean climate is characterized by the alternation of wet and dry periods within the year, with evident out-of-phase seasonal behavior of precipitation and temperature patterns
For the upper Alento River catchment (UARC), we evaluated the effects of seasonal anomalies by running Soil Water Assessment Tool (SWAT) simulations with synthetic rainfall time series considering different hypotheses of alternations of seasons, according to the static and the dynamic approaches described above
Capturing the relationship between precipitation and catchment-scale water balance components in a Mediterranean context is a scientific challenge in view of expected increasing frequencies in extremes such as droughts and floods induced by climate warming
Summary
The rainfall regime of the Mediterranean climate is characterized by the alternation of wet and dry periods within the year, with evident out-of-phase seasonal behavior of precipitation and temperature patterns. Summer is characterized by water stress due to scarce rainfall supply, combined with high evapotranspiration loss and the seasonal peak in water consumption (comprising agricultural, industrial, and recreational uses, hydroelectric power generation, and domestic uses, which are often boosted by tourism pressure). It is necessary to store water during the rainy period to cope with the uncertain duration of adverse water deficit conditions during the dry period. Water supply infrastructures necessitate high investment costs that strongly depend on the expected balance between the amount of water supplied in the rainy period and the amount of water lost and consumed during the dry season. The amount of rainfall in each season can be suitably decomposed and simulated on the basis of the following three main components: (i) duration of the seasons, (ii) occurrence probability of a daily rainfall event in each season, Published by Copernicus Publications on behalf of the European Geosciences Union
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