Storm intensity and duration impact on deep infiltration in rocky terrains

Anticipated future changes in precipitation patterns are expected to affect deep percolation (DP) through the vadose zone and groundwater recharge (GWR) of semi-arid regions such as Malta. Moreover, a diverse range of agricultural practices, from rainfed to irrigated agriculture, complicates the relationship between storm characteristics (magnitude, duration, intensity, antecedent dry spells) and DP. Variations in agricultural practices are often responsible for variations in wetness conditions within the vadose zone, which ultimately impact DP and GWR potential. To better establish this relationship, four years of deep vadose zone water content measurements obtained using a unique vadose zone monitoring system network across various agricultural land uses in Malta were utilised. Furthermore, the rainfall data over these four years were characterised into storms using minimum inter-event times (MITs) ranging from 12 to 168 hours. DP events in the vadose zone were identified from the VMS by detecting the first >1% absolute increase in volumetric water content at the deepest responding sensor within three days after the onset of each storm event. The optimal MIT selection was based on Cohen’s d effect sizes, which quantify how strongly each storm characteristic distinguishes DP-triggering storms from non-DP storms. MIT of 24 hours generally produces the strongest statistical link between storm characteristics and episodic DP across land uses. DP events typically occur during storms delivering approximately 30 mm of rainfall, lasting 1.5–2 days, and exhibiting peak intensities of 8 mm/h. Thus, it is the combination of event magnitude and rainfall duration that consistently distinguishes storms capable of generating DP. For rainfed agricultural land, which comprises more than half of Malta's agricultural landscape, DP is strongly controlled by storm magnitude and intensity, while also showing dependence on dry spell duration. However, in irrigated land uses, DP becomes less dependent on these storm characteristics due to elevated antecedent moisture from irrigation when compared to rainfed systems. Storm duration remains a moderately important factor in determining DP. Analysis of future rainfall projections for the SSP5-8.5 climatic scenario indicate reductions in storm magnitude and duration, together with longer dry gaps between storms. Ultimately, a decline in episodic DP frequency is expected in the rainfed agricultural land of rocky terrains in semi-arid climates such as Malta.