الملخص الإنجليزي
The Sultanate of Oman is an arid country where drought conditions prevail and water is precious. One of governmental efforts to augment water resources is by construction of recharge dams (e.g. Al-Khoud dam). Siltation is a common problem in many recharge dams around the world. Along with scaling-caking problem caused by siltation, a change in the hydrological properties (like reduction in infiltration) of the original soil porous media is common. Surface scraping is practiced to mitigate the scaling-caking problem along with improving the infiltration to some extent. In fact, scraping does not completely solve the problem of reduced hydraulic conductivity of the dam bed as fine soil particles may migrate into the deep subsurface layers which ultimately may alter the original alluvium characteristics. The main objectives of this study were: (i) to investigate the effects of anthropogenic activities on the subsurface soil physiochemical properties, and (ii) consequently, the impacts on hydraulic properties which are important for the augmentation of water resources of the underlying aquifers. To achieve the stated objectives, a number of field studies and laboratory experiments were conducted. A total of 33 soil pedons were excavated and described (15 inside; 18 outside the dam-12 of these in the recharge area downstream the dam). Physiochemical analyses include soil texture using Hydrometer method, measurements of total salts, soil pH and basic cations were also determined. Infiltration measurements were carried out using tension infiltrometers. In addition, several petrographic techniques like X-ray diffraction (XRD) and Scanned Electron Microscopy (SEM) were conducted to closer inspect the micromorphological and mineralogical composition of sub-soils in the study area. Major results of the study revealed that the sub-soil's physiochemical properties of the study area are changing due to human involvement in constructing the dam that result in changes in the hydropedological properties. Vertical variation in soil texture is evident in Al-Khoud dam recharge area which reduced hydraulic conductivity (e.g. Ks =0.01 cm/hr and 1.72 cm/hr for Pedons L and K inside the dam respectively) and hence, recharge into the underlying aquifers. Over time, and due to these changes, spilling of ponded water over the dam crest expected to occur more frequent and therefore, increases the potential hazards of flooding the downstream recharge area. In addition, pulses of finer particles .
might be sent to the soils of the recharge area where they may settle and even reduce the infiltration rate of these zones. Part of these finer particles may also migrate downward into the parent soil and cause clogging of large pores and hence, reducing subsurface conductivity (Ks= 0.37 m/day). Formation of incipient calcic layers and hard pans were also discovered in the study area which in turn affected the hydraulic measurements. The infiltration rate downstream of the dam was relatively high (6 m/day) - and it was 3 times higher than the average value inside the reservoir (2.1 m/day), ranging between 0.01 m/day to 3.96 m/day, but less than at the upstream site of pedon AG (outside the dam). Understanding both the dynamics of the soil particles and water in a subsurface system is of paramount importance for water resources management. This study presents a hydropedological approach in understanding the subsurface system in the vicinity of Al Khoud recharge dam in Oman and will assist in development of better managerial solutions and strategies in increasing dam efficiency.
