Revealing the interrelation among eco-hydro-meteorological variables in a forested Mediterranean catchment

Eco-hydro-meteorological variables (EHM) are key indicators for assessing the impacts of climate change on ecosystems, affecting hydrological processes and the resilience of forest systems. Meteorological forcing, such as precipitation and vapor pressure deficit, notably determines soil moisture variability, strongly related to tree transpiration and sap flow rate. In turn, soil moisture is affected by tree uptake. Understanding the feedback among these variables is crucial for effectively managing water resources and more robust predictions of the effect of climate change-induced droughts. However, few works have been conducted to disentangle the feedback of EHMs over time, frequency, and space domains in mountain forested catchments, especially in the Mediterranean region. To fill this gap, wavelet transform and coherence analysis were applied to investigate physical processes and their interaction in time and frequency domains.

We monitored EHMs for two years in a small sub-catchment (0.31 km2) of the Re della Pietra experimental catchment, Central Italy. The elevation and slope of the sub-catchment are about 940 m a.s.l and 36°, respectively, with a geological substrate of sandstones that promotes the development of well-drained sandy loam soils with a depth larger than 50-80 cm. The area is classified as a temperate Mediterranean climate with annual averages of 1300 mm for rainfall and 10.5 °C for temperature. The vegetation cover consists of pure beech forest. We monitored climatic variables with a weather station in the upper part of the sub-catchment, while soil moisture and sap flow variation were collected at different positions along a steep hillslope. We performed wavelet transform analysis to explore the EHMs variability over time, frequency, and space, while through wavelet coherence, we investigated the conditions and factors that influence the feedback dynamics of EHMs. 

Wavelet transform analysis highlights significant rainy periods exceeding the 1024-h frequency and a strong vapor pressure deficit seasonality, defining the alternation between dry and wet seasons. Soil moisture variability at the bottom slope position significantly differs from the upslope and midslope, and recovery periods following the dry season are more evident in the upperslope position than in the middle. High power values for sap flow at 12/24-h frequencies, revealing the daily tree transpiration, differ for the investigated positions. Wavelet coherence analysis remarks differences depending on the hillslope position. High coherence values between sapflow and soil moisture are shown for frequencies between 12/24-h for most of the tree growing season, with soil moisture driving sap flow. However, in the upslope position, the early stop of tree transpiration caused by sharply reduced soil moisture resulted in low coherence values. High coherence values are also highlighted for frequencies larger than 24-h, showing the sap flow leading to soil moisture. Sap flow strongly correlates with vapor pressure deficit in all frequencies of the monitored period, while coherence with precipitation is significant only for frequencies greater than 64-h.

Through the application of wavelet analysis, this study presents an in-depth investigation of the complex relationships between eco-hydro-meteorological dynamics in forest catchments.