Hydropedology and soil evolution in explaining the hydrological properties of recharge dams in arid zone environments

The effects of anthropogenic activities on soil evolution due to siltation and the impact on hydraulic properties of the vadose zone on the augmentation of the aquifer recharge are investigated for the Al-Khoud dam in Oman. Inside the dam reservoir and in areas adjacent to the embankment, downstream, 33 pedons (of depths between 1.5 and 2 m) were excavated and studied during 2011 and 2012. Soil analysis revealed that the subsoil’s physiochemical properties of the study area are continuously changing due to damming, i.e., alteration of the natural runoff, intensified sedimentation, and infiltration. Variation of hydropedological properties caused by the geotechnical construction is evident in a distinct vertical stratification of texture of accrued sediments and almost an order of magnitude drop in saturated hydraulic conductivity (K_s) of the dam bed. Correspondingly, spilling of ponded water over the dam crest occurs more frequent and therefore increases the potential hazards of flooding of the downstream recharge area. Some fine particles of the suspended load carried to the reservoir by the feeding wadi migrate vertically downward, driven by seepage, into the originally coarse matrix of the parent soil and cause clogging of large pores (with time, hard pans in the subsurface are developing) even without visible cake formation on the soil surface. Development of hard pans was also discovered in pedons at depths close to 1 m. This is attributed to presence of a pedogenic carbonate derived from the parent rock and formed by precipitation of dissolved salts due to a vertical upward moisture evaporation to a hot and dry bed surface during prevailing dry bed periods of dam operation. K_s measured downstream of the dam was relatively high (6 m/day) and was three times higher than the average value inside the reservoir (2.1 m/day), ranging there between 0.01 and 3.96 m/day, but less than at the upstream site outside the reservoir.