Dear friends, I have got some replies to my question on how to estimate soil humidity from topographic or texture data. Here they are:
************** If you are interested in how much moisture the soil can hold than soil texture is your best proxy.Soil humidity is hard to measure anyway, since it fluctuates throughout the year depending on rain and temperature. Soil texture, however, can tell you more about the water retaining properties of the soil. If you know Organic Matter and Texture you can estimate Field Capacity based on equation by Saxton and Rawls (2006). I have the paper saved somewhere on my computer, I can send it to you if you like. Hope this helps. ************* There are a number of reasonably well documented equations and soil moisture model (or "Topographic Wetness Index, TWI) approaches for calculating soil moisture parameters using a raster digital elevation model. The calculations (which I'm guessing have evolved over the past few years) are simply a function of the slope and the contributing upslope area; with the basic understanding that, all things (such as land cover and soil catenas) being equal, the bottom of a long slope slope will tend to be wetter than the top of a short slope. Some of the older hydrological software addons to ArcGIS (TauDEM, etc...) even have Soil Moisture functions included. The simplistic notation is: LN (a/S) or the natural log of the Upslope Area divided by the slope (for all points\pixels within the DEM). H. Mitasova has been working on the geospatial estimation of such estimates (which can easily be built into scripted functions within packages such as ArcGIS with Spatial Analyst (as well as Open Source software such as GRASS)... For more see: http://naldc.nal.usda.gov/download/44278/PDF http://skagit.meas.ncsu.edu/~helena/gmslab/reports/CerlErosionTutorial/denix/Advanced/ErosionRep97/rep97.html http://hydrology.usu.edu/RRP/userdata/4/87/Appendices.pdf http://ks.water.usgs.gov/pubs/reports/wrir.99-4242.html http://journalofmaps.com/student/10_01_Hardy.pdf
