## 2 Materials and Methods  
### The Study Area  
Al-Ahsa oasis is an area distinguished by date palm cultivation, located in the Eastern province of the KSA about 70 km to the west of the Arabian Gulf coast, 150 km southwest of Dammam port, and 320 km to the east of the capital city Riyadh. It lies between longitude of 49\({}^{\circ}\)30\({}^{\prime}\)-49\({}^{\circ}\)50\({}^{\prime}\) E and latitude 25\({}^{\circ}\)20\({}^{\prime}\)-25\({}^{\circ}\)40\({}^{\prime}\) N (Figure 1), with an elevation of 160 m above sea level in the west that gently inclines toward the Gulf coast [28]. The oasis generally has an \"L\" shape with an axis extending for 30 km in the south-north direction and the other axis stretching for 18 km in the west-east direction. Both axes intersect in the southwest corner of the oasis at Al-Hofuf city, the largest metropolis in the area [17]. Studies have also pointed out that the oasis is primarily surrounded by active and mobile sand dunes originated from Al-Jafurah desert, which initiates sand encroachments at northern, eastern, and southern boundaries jeopardizing the oasis from the north-west and north routes [18; 29].  
The climatic data considerably vary between seasons and years in the Al-Ahsa oasis. Based on Mansour [30], the oasis is situated in the sub tropic arid zones, with a climate generally characterized by its very hot-dry summers and cool and relatively dry winters. According to [PERSON] [31], the summer season starts in May to the end of August with a mean temperature ranging from 45 \({}^{\circ}\)C to 24 \({}^{\circ}\)C, while the winter season begins in November to the end of March with a mean temperature ranging from 21 \({}^{\circ}\)C to 8 \({}^{\circ}\)C. In summer, the area is considered as the hottest region in the country. The relative humidity varies between 21% and 29% in summer and between 31% and 55% in winter, with a monthly average of 38%. The rainfall is usually in winter and spring seasons with an overall annual mean of 85 mm.  
The total area of the oasis is about 20,000 ha, among which more than 8200 ha are under irrigated cultivation, with 92% being planted with date palm trees [17]. The date trees in the oasis are usually planted at 6 m \(\times\) 6 m distances, with interspaces being planted with fruits, vegetables, or field crops, depending on the space between the date trees [17; 31]. The cultivated area is practically divided into three subregions, namely: the northern (i.e., Jlejiah), the eastern (i.e., Al-Mansourah), and the central (i.e., Al-Harah). The soils of the cultivated lands in the oasis are typified with spatially varied properties that decline their quality and hence productivity. These properties mainly include prevailing light texture, high salinity and calcium carbonate contents, and low contents of organic matter and essential plant nutrients [32]. These soils also are dominated by the six great groups, which are Salorthids, Gypsiorthids, Calciorthids, Psammaquants, Haplargids, and Torripsamments [33].  
The main source for irrigation water is groundwater, which is mainly supplied from the karistified neogene aquifer (i.e., 100-180 m deep) that belongs to the Umm-er-Raduma formation [34; 35; 36]. Such water is characterized by high (C3) to very high (C4) salinity hazards and low (S1) to medium (S2)odicity hazards [37; 38]. Additionally, water properties were verified by other investigators [39], who also implied that these properties have spatial variations with salinity and its components increasing in the north and north-east directions.  
Figure 1: Study area map, where Al-Ahsa oasis is delineated in the blue and the distribution of the representative farms is marked in the green.