Impact of slope inclination on salt accumulation
Introduction
Soil salinity is a serious environmental problem especially in arid and semiarid areas. Its occurrence is either natural or human-induced. High levels of soil salinity have negative effects on crop growth and productivity, resulting into land degradation ultimately (Allbed and Kumar, 2013).
Salt accumulation in soils is being driven by hydrological processes, which cycle seasonally, but also changes as a result of climatic changes over a long period of time, and changes in land use and land management practices (Nachson et al., 2014).
Essentially, soil salinity is a dynamic process that poses severe consequences on the soil, hydrological, geochemical, agricultural, social and economic facets. In order to cope with further salinization and to sustain agricultural lands and natural ecosystems, information on the spatial extent, nature and distribution of soil salinity is becoming of great concern (Allbed and Kumar, 2013).
Field observations have shown high variability in soil salt concentration along different slopes. And this led to the quest to understand if there’s a correlation between slope inclination and the nature and location of salt accumulation. And if yes, why?
Hypothesis
It was hypothesized that for low inclination slopes capillary flow is mainly perpendicular to evaporation front therefore salt accumulates homogeneously along slope surfaces. Lateral transport of water and solutes is mainly within the saturated zone - far below evaporation front. However, for high inclination slopes capillary flow is mainly parallel to evaporation front which leads to salt transport towards the upper boundary of capillary fringe.
Materials & Methods
To examine the impact of slope on salt accumulation, we measured salt accumulation along a 50cm long slope (column) at different inclinations. Hele-shaw column was filled with fine sand and solution was of 15% NaCl.
Furthermore, to test hypothesis and to determine water flow paths along inclined slopes at different inclinations, we examined colored water flow paths and velocities in inclined Hele- Shaw cells at various inclinations. Water supply was from the lower left corner.
Results
It was observed that as inclination increased salt accumulation along slope surface shifted upward along the slope, thickness of salt layer reduced and salinity increased.
For 0 degrees inclination conditions, a non-uniform flow velocity profiles were observed. Maximal velocity was observed at the saturated zone and is attributed to the high hydraulic conductivity of that zone. Lowest (lateral) velocity observed just above water table. Above that region velocity increases again, probably due to evaporation water consumption.
Furthermore, for 45o inclination a uniform flow velocity profiles were observed, indicating on uniform transport of solutes (or color) along the slope.
Conclusion
Results showed that inclination affects salt accumulation along slopes. As inclination increases salt accumulates higher along the slope, with higher concentrations and within a narrower band. This is due to differences in subsurface flow paths - associated to slope inclination.
