Root-induced changes of soil hydraulic properties – A review

Regional-scale hydrological investigations that involve vegetation change usually neglect root-induced modification (RIM) of soil hydraulic properties (SHP). However, there is evidence that root distributions may impart major control over SHP. A critical barrier to incorporating RIM of SHP in catchment hydrology models is the high uncertainty of root-effect parameterization. In response to this, the current article reviews available literature to summarize the key processes, measurement, prediction and implications of RIM of SHP. Previous studies found contrasting root effects (on SHP) depending on which processes are dominant, including root growth (or decay) and the density and diameter of roots. Based on data from laboratory experiments and field testing, RIM of SHP can be summarized as: (1) fine roots (root diameter < 1 mm) at low density tend to block soil pores, leading to reduced saturated water content (θ_s), saturated hydraulic conductivity (K_s), and higher air entry potential (AEP); (2) fine roots at high density tend to eliminate macro-pores while increasing pore volume, leading to increased θ_s and AEP, and reduced K_s; (3) coarse roots (root diameter > 2 mm) tend to cause small-scale compaction, particle re-orientation and macro-pore development, leading to increased θ_s and K_s, and decreased AEP. Root growth also tends to enhance soil moisture retention hysteresis. Root decay is expected to have the opposite effects to root growth due to the creation of continuous macro-pores, which may lead to K_s increases of up to two orders-of-magnitude. In general, the enhancement of K_s due to plant roots can be expected in fine-textured soils, whereas plants roots are known to reduce K_s and θ_s and increase soil water retention capacity in coarse-textured soils. An important aspect of RIM of SHP is the temporal variability that arises from seasonality and/or agricultural cycles. While neglecting RIM of SHP in hydrologic simulation may lead to high uncertainties, there remain significant knowledge gaps that impede the development of quantitative guidance on RIM of SHP at the catchment scale, at which variability in environmental conditions and human activities precludes the application of simple root-effect equations.