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Model for the Soil Solution Composition of an Oasis¹

ABSTRACT

A static computer model based on the evaporative concentration of irrigation waters has been developed for the origin of soil solution compositions in the Al-Hassa oasis in Saudi Arabia. The model successfully predicted the precipitation of calcite and the conversion of smectite to attapulgite when in equilibrium with atmospheric CO₂ pressure. It also predicted precipitation of gypsum when irrigation water is concentrated by a factor of 3.7 and the precipitation of silica gel when water is concentrated by a factor of 7.5. Results from this study indicate that the model may be applicable to other oases in arid environments. Spring water discharges freely from the higher elevations and flows toward playas in the desert. Mineral fractionation follows the same natural slope within the oasis, as indicated by isoconcentration lines for salinity, carbonates, and gypsum in oasis soils. The calculated soil solution compositions at fixed electrolytic conductivity (EC) values were compared with the chemical analysis of 1:5 extracts having the same EC values. These comparisons showed that six primary processes are responsible for the formation of soil solutions from irrigation water under oases conditions. The processes are: (i) deaquation due to transpiration and evaporation, (ii) precipitation and dissolution of soil mineral phases, (iii) K fixation, (iv) Na and Cl retention, (v) MgSO⁰₄ formation, and (vi) biological formation of HCO₃. Agricultural production of dates was absent in areas where the EC of 1:5 soil solution extracts exceeded 6 dS m^–1 (i.e., in gypsum and silica gel precipitation areas).

¹ Contribution from the Dep. of Soils and Water, King Faisal Univ., Al-Hassa, Saudi Arabia. Supported by the Saudi Arabian National Center for Science and Technology (SANCST) under contract AR-1-018.

² Professor of Soil Science, Dep. of Soils and Agricultural Chemistry, Univ. of Holwan, Alexandria, Egypt.

Received for publication September 8, 1983. Accepted for publication December 2, 1981.