Rangeland Soil Water Balance
By Dr. Lynn Loomis
During the past two years, Alamito Foundation has developed infrastructure to measure various components of the rangeland soil water balance on Alamito Ranch. By measuring the behavior of water, we can begin to observe the hydrologic consequences of various land management practices.
A water balance is the account of the inputs and outputs of water at a place. The balance can be determined by measuring or computing calculating water inputs, outputs, and changes in storage.
P + O = R + I
Inputs of Water
P = Precipitation is the addition of water, in the form of rain, snow, hail, to the site.
O = Run-on water is the addition of water derived by surface runoff from upslope areas.
A one-inch rain, distributed evenly across the 3500-acre Alamito Ranch, adds about 100-million gallons of water. Weight of that added water is more than 440,000 tons. According to a PRISM normal precipitation grid, annual rainfall at Alamito Ranch is about 15 inches (about 1.5 billion gallons of water).
Precipitation inputs are controlled by atmospheric processes. While the type, amount, intensity, and seasonal distribution of precipitation do affect the balance of infiltration and runoff, rainfall itself is not subject to modification through management.
Outputs of Water
R = Surface Runoff
I = Infiltration
Water added to the soil surface experiences one of two outcomes: infiltration or surface runoff. When added at a rate less than soil infiltration capacity, water passes through the soil surface and enters pore space in the soil. No runoff occurs. The force of gravity pulls the water downward through the network of soil pores. Water adhering to the surface of soil particles is available for uptake and use by plants.
In contrast, surface runoff occurs when the precipitation rate exceeds the soil infiltration capacity. Excess water first ponds in depressions on the soil surface, then spills out of depressions, eventually flowing down slopes, and ultimately gathering in channels.
Unlike the input components of the water balance, output components of the water balance are subject to modification through management. A minimum quantity of litter and vegetation cover is necessary to protect the soil from raindrop impact and overland flow. Grazing management controls the amount of standing vegetation and plant litter remaining on the soil surface. When grazing animals reduce litter and standing plant material below that minimum, the soil surface is exposed to the kinetic energy of falling raindrops and turbulent concentrated surface flow.
Outputs continued
Water that infiltrates the soil can be lost via three pathways.
I = Infiltration
I = E + T + G
Outputs
E = Evaporation
T = Transpiration
G = Groundwater Recharge
Evapo-transpiration is a measurement of the total amount of water needed to grow plants and crops. The term is a contraction of the words evaporation (i.e., loss of water directly from the soil) and transpiration (i.e., loss of water from plants).
Potential evapo-transpiration (pET) is the amount of water that could be evaporated under most favorable conditions, including a limitless water supply. pET was computed from GIS grids of monthly temperature using the Thornthwaite method. According to a pET GIS grid, annual water loss from Alamito Ranch is about 34 inches. The annual water deficit (annual rainfall minus annual pET) at Alamito Ranch is about 19 inches. That is, the ranch experiences 19 inches more of water loss in a year than is added by rainfall.
Soil Data
John Jacob, for his MS thesis at Texas Tech University, described soil morphology of three profiles on Alamito Ranch in 1983. John collected samples from each soil horizon and determined their physical, chemical, and mineralogical properties in the lab. Soil properties affecting the movement and storage of water included clay content, organic matter content, bulk density, water content at 1/3 and 15 bar potential, as well as available water capacity. Using data on porosity and water retention archived in John’s thesis, I computed, for each of the three profiles, how deep wetting fronts would penetrate the soil resulting from 0.1-inch, 0.25-inch, 0.5-inch, 1-inch, and 2-inch rainfall events.
TWDB soil/water monitoring station
In February 2022, working together Texas Water Development Board (TWDB) and Alamito Foundation staff installed a soil/weather observation station on Alamito Ranch. The station was placed at one of John Jacob’s thesis pedons. Instruments of the station continuously measure 1) soil moisture, 2) soil temperature, 3) precipitation, 4) air temperature, 5) dewpoint temperature, 6) solar radiation, 7) wind speed, and 8) wind direction. At 5-minute intervals, data values are uploaded via cellular network to a TWDB website and archived. The soil and atmospheric data collected since March 2022 are available for download by the public.
TexasET network
The Texas A&M AgriLIfe Extension Service TexasET network ingests data reported by the TWDB soil/weather station, then computes the value of evapo-transpiration (reference ET) daily. In addition to the effect of air temperature on water loss, the Penman-Monteith method used by TexasET incorporates the effects of solar radiation, relative humidity, and wind.
