Estimation of potential nutrient fluxes from the Wadi Gaza catchment into the Mediterranean Sea with emphasis on flooding events

The lowland areas and coastal seas may receive massive volumes of water and associated nutrients loads during flood periods (Al-Najjar et al. 2022; Ziouch et al. 2020; De Girolamo et al. 2017). Even though the Mediterranean catchments are expected to experience severe droughts with climate change, flush floods might also increase in intensity and frequency resulting in significant episodic water and nutrient transports (Al-Najjar et al. 2020; Zettam et al. 2020). In the Mediterranean Sea, riverine flooding is particularly important for maintaining marine productivity, where the annual average nutrient loads discharged into the Mediterranean from the inland catchments were estimated as 1.87 million tons of TN, 1.22 million tons of N-NO₃, 0.11 million tons of TP and 0.03 million tons of P-PO₄ (Malagó et al. 2019). The catchment of Wadi Gaza is one of the major east Mediterranean catchments which provides the Gaza Strip in Palestine with a seasonal water stream from the Hebron Mountains. The hydrological analysis of the Wadi Gaza water resources is a significant challenge to evaluating the functioning of the land-sea interaction and understanding the water and nutrients fluxes into the Mediterranean coast (NAP 2016). The nutrients transported by the catchment of Wadi-Gaza to the downstream zone of the Mediterranean may reach approximately 2,000 tons of TN and N-NO₃, as well as 5 tons of TP and P-PO₄ per annum, according to a recent modeling of the terrestrial nutrient fluxes into the Mediterranean Sea (Malagó et al. (2019). Additionally, the Wadi Gaza basin, a transnational watershed governed by international water management guidelines, is experiencing pressures due to urbanization and climate change, which disrupt the basin's natural processes and have an impact on the habitats of about 154 faunal and 70 floral species (NAP 2016). The natural dynamics of Wadi Gaza, similar to those of many catchments in the eastern Mediterranean, are still uncertain and need to be investigated by conducting a simulation model for the entire basin. However, throughout the basin of the Mediterranean Sea, many researchers demonstrated the hydrological response of the Mediterranean catchments (Zettam et al. 2020; De Girolamo et al. 2017; Cooper et al. 2013; García-Pintado et al. 2007; Lillebø et al. 2007). A well-calibrated catchment model would be an essential tool for the water management authorities to better and more sustainably manage the water resources and to estimate the quantity and quality of flowing water in Wadi Gaza. The soil water assessment tool (SWAT) is a robust hydrological tool that is used to model the flows, sediment loading, and nutrient content of the water streams based on the setup of the functional units of land use, soil, and slope (Nguyen et al. 2019; Liu et al. 2017). In particular, this study provides an assessment and evaluation of the hydrological features of Wadi Gaza in Palestine by applying the SWAT model to predict the surge of water quantities and the loads of nutrients that are transported to the Mediterranean Sea.

The dataset of the daily rainfall was accumulated into the form of the total monthly rainfall and analyzed in terms of trend using Mann–Kendall and Sen slope estimators. A digital elevation model (DEM) and satellite image covering the study area on a cell resolution of 30 m were obtained from the United States Geological Survey (USGS) (https://earthexplorer.usgs.gov) and the soil map was extracted from the soils portal of the Food and Agriculture Organization (FAO) (https://www.fao.org). The satellite images were processed on the platform of ArcGIS for producing the land use map of Wadi Gaza. The SWAT model (Arnold et al. 1995, 1998, 1999) was used in this study to process the hydrological assessment of Wadi Gaza to focus on the stream flows and the sediment and nutrient loads discharged to the downstream zone where the SWAT model depends on the stratification of the input layers and detecting the data from each of the layers based on the resolution of the layers, in parallel, the model delineates the study area according to the accumulation of the flow from the upstream to the downstream. The rainfall daily data and the surface runoff discharges were studied using the Gamble frequency analysis method to specify the extent of the inundation area downstream of Wadi Gaza. Admittedly, there are no measurements of the fluxes and transported nutrients downstream of Wadi Gaza, which makes it difficult to calibrate and verify the model. However, the need for catchment-specific analysis makes modeling processes valuable for illustrating the hydrological processes of the water basin. Hypothetically, considering that there is no monitoring program for flux gauging or sampling campaigns for nutrient estimations, particularly in the eastern Mediterranean catchment areas, and since the majority of studies that have been published for the entire Mediterranean catchments rely on metrological-based simulations, it is appropriate to introduce an assessment study by relating the fluxes of the streams with the potential sources of nutrients. This strategy is scientifically promising for the decision-makers to enable estimating the quantities of fluxes and nutrients which is crucial for the economic and agricultural growth as well as for understanding the behavior of flooding in the area for management and preparation of evacuation and flood alarming system based on the buffer zone of the flood.

The statistical analysis of the meteorological data of rainfall, as depicted in Table 1, reveals that the total monthly rainfall ranges between a minimum and a maximum value of zero and 445 mm, respectively. Further, the mean total monthly rainfall over the basin of Wadi Gaza ranges between 13 and 200 mm. The trend analysis of the monthly rainfall in terms of the Mann–Kendall test and Sen's slope estimator refers to a non-trend or a decreasing trend in the pattern of the rainfall.

The frequency analysis was performed using the Gamble method over the return periods of 2, 25, 50, and 100 years for the rainfall and the surface runoff as shown in Table 2.

Potentially, an area of about 2.5 km² is under serious threats of flooding every 2 years while every 100 years, the inundation area is expected to be about 15 km² of land. In this context, the stream runoff flooding of 18, 91, 109, and 127 m³/s might provide the Mediterranean ecosystem with nitrogen inputs, respectively, of about 1,142, 5,773, 6,915, and 8,057 tons of TN. In addition, these discharges are expected to transport a phosphorus source of about 414, 2,092, 2,505, and 2,919, respectively.

The hydrological catchments of the southeastern Mediterranean are vital sources of water and nutrient supplies for the marine ecosystem. However, the lack of studies about the hydrological response of these catchments is an alarming matter in light of climate change implications, anthropogenic processes, and the recorded events of flooding. Therefore, the simulation-based assessment of the southern Mediterranean catchments is an effective trend to demonstrate the conditions of the hydrological features in terms of many affecting factors. The SWAT model is an efficient tool to study the hydrology of the Wadi Gaza catchment to define the potential resources of this area as well as to evaluate the flooding incidents. According to the simulation study for the catchment of the Wadi Gaza, the following could be concluded:

• On average, the stream downstream discharges vary between zero in the dry months and 15 m³/s in the wet season.

• During the rainy seasons, there may be as much as 3,673, 177, and 62 tons of sediment, TN, and TP, respectively, that could be transported to the Wadi Gaza stream's downstream region.

• Every 2 years, there is a major risk of flooding in an area of about 2.5 km², and in 100 years there will likely be an area of around 15 km² under water.

• Efficient flood management and alarm systems for the downstream part of the Wadi Gaza basin should be designated to prevent sudden surges of flooding leading to disasters.

In particular, the runoffs of the surface water from the flooding of the Wadi Gaza basin are rich in the nutrient that might be used for enhancing the agricultural irrigation system in the Gaza Strip beyond using synthetic fertilizers that might negatively impact the environmental system. Furthermore, the runoffs comprise a considerable amount of water that might be used for closing the gap in the groundwater deficit and the water sector.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.