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Interaction between soil pedogenesis, subsurface water dynamics, climate, vegetation and human ingenuity in a desert environment has been found to result in a unique ecohydrological system with an essentially three dimensional sedimentation structure in the bed of a recharge dam in Oman. A 3-D array of silt blocks sandwiched by dry sand-filled horizontal and vertical fractures was studied in pot experiments as a model of a natural prototype. Pots are filled with a homogenous sand-silt mixture (control) or artificially structured (smart design, SD) soil substrates. Rhodes grass and ivy (Ipomea, Convolvulaceae) were grown in the pots during the hottest season in Oman. Soil moisture content (SMC) was measured at different depths over a period of 20 days without irrigation. SD preserved the SMC of the root zone for both ivy and grass (SMC of around 25%-30% compared to <10% for control, 3 days after the last irrigation). Even after 20 days, SMC was around 18% in the SD and 7% in the control. This, similar to the case of a natural prototype, is attributed to the higher upward capillary movement of water in control pots and intensive evaporation. The capillary barrier of sand sheaths causes discontinuity in moisture migration from the micro-pores in the silt blocks to sand pores. The blocks serve as capillarity-locked water buffers, which are depleted at a slow rate by transpiration rather than evaporation from the soil surface. This creates a unique ecosystem with a dramatic difference in vegetation between SD-pots and control pots. Consequently, the Noy-Meir edaphic factor, conceptualizing the ecological impact of 1-D vertical heterogeneity of desert soils, should be generalized to incorporate 3-D soil heterogeneity patterns. This agro-engineering control of the soil substrate and soil moisture distribution and dynamics (SMDaD) can be widely used by desert farmers as a cheap technique, with significant savings of irrigation water.

A pedological study of the reservoir bed of Al-Khoud Dam, Oman, revealed an unusual sedimentation pattern which evolved into an intricate composition of silt blocks surrounded by vertical cracks and horizontal layers filled with a "proppant" sand. The discovered soil morphology reflects the complex topology of water motion (infiltration-seepage-evaporation) through the sand-filled cracks/layers and blocks during both the rare flood events and ensuing periods of ponding, and the long, intervening dry periods. These naturally formed soils demonstrate an ability to preserve a large quantity of water inside the silty blocks at depths of 0.5 to 1.5 m, despite the high temperature and dryness of the topsoil. The hydrological optimality and "smartness" of these soils is attributed to the unique block-crack system. Natural, lush vegetation was found in adjacent zones of the reservoir bed, and acted as a footprint of the shallow "fractured perched aquifer". Planted "ivy" (Convolvulaceae) in the vertical face of one pedon showed intensive growth without irrigation. Soil moisture content data confirmed the hydrological immobility of water in the blocks if not depleted by transpiration. The novel phenomena reported unveil the possible alteration of soil heterogeneity for optimization of the soil-water system in arid zone soils.Key words hydropedology; capillary barrier; arid zone; 3D block-fracture composition; evapotranspiration; infiltration Sédimentation préférentielle en blocs et fissures: structure originale et évolution naturellesRésumé Une étude pédologique du lit du réservoir d'Al-Khoud, Oman, a révélé un mode de sédimentation inhabituel conduisant à une formation complexe de blocs de limon entourés de fissures verticales et d'ouvertures horizontales remplies de sable jouant un rôle d'agent de soutènement. La morphologie des sols découverts reflète la topologie complexe des mouvements de l'eau (infiltration-percolation-évaporation) à travers les blocs et les fissures pendant les rares épisodes de crues suivis de la formation de flaques superficielles puis enfin au cours des longues périodes de sécheresse habituelles. Ces sols formés naturellement ont démontré une capacité à conserver une grande quantité d'eau à l'intérieur de leurs blocs limoneux à une profondeur de 0,5 à 1,5 m, malgré la température élevée et la dessiccation de leur surface. La configuration du sédiment doit ses propriétés hydrodynamiques à des barrières capillaires. L'optimalité hydrologique et « l'astuce» de ces sols sont attribuées à l'existence d'un unique système matriciel de fissures. Signe de l'existence d'un « aquifère perché fracturé » superficiel, une végétation naturelle luxuriante a été observée dans les zones de réservoirs voisines. Une plantation de lierre (Convolvulaceae) sur la face verticale d'un pédon a connu une croissance intensive sans irrigation. Les données de teneur en humidité des sols ont confirmé la rétention de l'eau dans les blocs si elle n'est pas prélevée par la transpiration. Les nouveaux phénomènes rapporté...

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