Investigating hydrological dynamics of the agricultural catchment draining to Wied il-Qlejgħa : a comprehensive monitoring approach

Abstract

This study aims to assess the hydrological features of Wied il-Qlejgħa using integrated monitoring. Wied il-Qlejgħa and its upstream sub-catchments are situated within the Għasel Catchment System in a region which is predominantly characterised by natural and agricultural areas. This valley was chosen due to the current monitoring infrastructure available and its potential for adopting a combined monitoring methodology. The research employed manual stream gauging, water level measurements, gravimetric soil moisture monitoring and drone surveys, alongside GIS interpolation, to aid in the determination of critical hydrological attributes and relationships within the area of interest. Combined with climatic data, these results were used to estimate runoff coefficients for selected flow monitoring sites and the proportion of water lost as evaporation and recharge for the studied basins. Similarly, hydrological data were also used to aid understanding of rainfall-runoff relationships. When evaluating results, it was evident that there was a strong relationship between precipitation levels and discharge rates at manual stream gauging sites. Yet, factors such as soil type, topography, vegetation and rainfall intermittence influenced discharge rates to varying degrees. In-situ soil samples demonstrated water content variations related to rainfall patterns, with soil type and land cover affecting soil water content measurements. Additionally, remotely sensed products portrayed a clear shift from high conditions during wet periods to declining trends as a result of a lack of rainfall. Runoff coefficients were usually relatively low at manual stream gauging sites and negligible for most of the field visits to monitored basins. As a result, water retention was lacking through most of the period of interest, consequently leading to varied recharge rates across the three monitored basins affected by both their catchment area and underlying geology and geomorphology. Evaporation rates increased steadily during data collection in relation to the surface area of the basin. Both low and high flow stations generally displayed rapid responses to minor and major precipitation events at both sites, producing varying discharge rates. Nevertheless, actions such as water withdrawal were observed throughout the study, directly impacting field measurements. As many studies have several limitations, such as inadequate soil sampling locations and inaccurate flow measurements, this data was also noted. The integrated monitoring approach seems to have successfully characterised the hydrological behaviour of Wied il-Qlejgħa and its upstream catchments, whilst providing insights into water balance components and rainfall runoff relationships.