Soil substrate as a cascade of capillary barriers for conserving water in a desert environment: lessons learned from arid nature

Abstract

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.

Moisture and temperature in a proppant-enveloped silt block of a recharge dam reservoir: Laboratory experiment and 1-D mathematical modelling

“…VMC curves (Fig. 3a) have periodic blips, which reflect diurnal variations of the moisture content due to sorption-desorption of air humidity, as we witnessed in the farm experiments (Al-Maktoumi et al, 2014). As is evident from the temperature curves in (FIg.…”

“…Our focus is on the desiccation stage in (Fig. 3a), which is most important for agronomic and hydroecological applications (Al-Ismaili et al, 2013, Al-Maktoumi et al, 2014 because the blocks, isolated by the capillary barriers, do not drain, i.e. serve as excellent storage compartments of soil water, which in the field is consumed by the plant roots.…”

“…Consequently, the whole cascade of the soil structure is a triple-periodic composite, with sharp contrasts of hydraulic and thermal properties between texturally contrasting components (blocks and filled fractures). Al-Ismaily et al, (2015), Al-Maktoumi et al ( 2014), Al-Saqri et al (2016) studied further hydrological and geotechnical applications of natural "smart design" patterns.…”

“…The discovered "smart design" of the soil structure was replicated in an on-farm experiment and showed an excellent water saving efficiency for crop cultivation in Oman (Al-Maktoumi et al, 2014) and in growing ornamental plants as passive thermal coolers of building envelopes (Kacimov et al, 2010). The objective functions in the agroengineering design and optimization included biomass, yield and leaf-area index.…”

Research-based learning for undergraduate students in soil and water sciences: a case study of hydropedology in an arid-zone environment

“…program with an RBL exercise closely linked to research interests of the course instructors Al-Maktoumi et al, 2014). The RBL exercise reported here therefore also serves to develop the research-teaching nexus by connecting students with staff research interests (cf.…”