## 3 Earth Surface Modelling Advances and Links with EO Datasets
### Land Surface Properties
#### 3.3.3 Soil Texture  
Soil properties, particularly the texture of the soil in the near-surface horizon, obviously affect the evolution of soil moisture by controlling the ability of water to move in the soil column. Water retention and conductivity, as governed by soil texture, have secondary effects such as controlling the persistence of soil moisture anomalies and helping determine what type of vegetation can flourish in a location. Less evident is how subsurface properties affect surface soil moisture evolution. This is not well appreciated because it is very difficult to discern the structure of the terrain below the top meter or two of the soil. Weather and climate models typically treat the vadose zone below vegetation roots as terra incognita, assuming no spatial variability except that related to the slope of terrain. If a model simulates the water table, it is often connected directly to the shallow soils without regard to the spatial variability in between. Lumped hydrologic models infer the effect of the vadose zone in their calibration, but this is not possible in ungauged or arid basins. In reality, there can be preferential pathways of drainage that quickly remove water from the reach of direct evaporation or plant transpiration [216]. These correspond to areas of fractured or unconsolidated bedrock, called karst that underlie a significant fraction of the land surface, about a quarter of the continental US [217] and western part of Kazakhstan, where landscape can change quite dramatically, including emergence/disappearance of lakes, in a very short period [218]. Observational and modeling evidence suggests regions overlying shallow karst formations experience very different soil water evolution, affecting surface moisture and heat fluxes, vegetation distribution, and even impacting the evolution  
Figure 18: Global map of soil and alluvial thickness [213]. of convective precipitation [219]. A direct way to discern the effect of karst is to examine time series of in situ soil moisture measurements from networks using consistent instrumentation spanning both karst formations and less permeable bedrock. In the southern Appalachian region around the Middle Tennessee River Basin, there are extensive shallow carbonate karst formations and a fairly dense network of US Department of Agriculture (USDA) soil moisture stations spanning karst and non-karst substrates. A strong correspondence between weak soil moisture memory (lagged autocorrelation of anomalies) and the presence of karst has been found across a dozen stations [220]. Comparison among closely-located stations helps control for the confounding effects of other climatic, vegetative and hydrologic factors. The methodology developed for in situ data has been applied to remotely sensed soil moisture georeferenced against high-resolution karst information from the US Geological Survey [217] across the southeastern and south-central US. Small-scale (within 2700-3000 sq.km) spatial variability in karst coverage correlates significantly with small-scale variability in soil moisture memory [221]. Figure 19 shows that, where there is strong spatial variation in karst coverage, there is also greater variation in soil moisture memory as derived from NASA/SMAP satellite soil moisture. Correspondingly, where there is little or no variation in bedrock permeability, soil moisture memory is also more uniform. The fact that the signal of karst appears in satellite soil moisture data yields promise that remote sensing could contribute important hydrologic information in poorly-observed regions of the globe. Radiances in near-infrared and other parts of the electromagnetic spectrum have been shown to correlate to surface soil properties in unvegetated areas [222; 223]. Remote sensing of soil moisture is achieving levels of quality and information content that can allow inference of surface and subsurface properties otherwise unobserved or unavailable.