Conserving Soil Water through Grazing Management
Chad L. Engels, Graduate Student, NDSU Department of Civil Engineering
Rainfall is a major limiting factor for livestock production from western rangelands. This means simply that given all the factors that affect the health and growth of grasslands (e.g. nutrients, temperature, light energy, etc), the factor which tends to short circuit growth first is available water. This has led to an interest in managing grasslands for water conservation, which in turn means managing for optimum production. Other benefits of thinking in terms of water conservation are reduced erosion, reduced nutrient losses, and improved water quality. Figure 1 shows a conceptual model of water use on a rangeland. This is an approximate use generalization which shows that for 20 inches of annual rainfall, only 1.1 inches is used to produce forage that is actually consumed by livestock. Knowing the variables that reduce the water available for forage production (evaporation, runoff, deep percolation, undesirable plants, etc), we can develop methods/practices to target these specific variables. The focus here is runoff.
If rainwater has not taken the form of runoff, or has not evaporated, then it has infiltrated into the ground. This is the pathway we desire to achieve. But before we can take action to increase the amount of rainwater that infiltrates into the soil, first we must understand exactly what infiltration is. Infiltration is the passage of rainwater into the soil surface, as opposed to percolation, which is the movement of water through a soil profile. The rate at which water passes into the soil surface is typically quantified in inches per hour. Note that this is also how rainfall intensity is quantified. For example, let's assume that we have a rangeland with a maximum potential infiltration capacity of 0.5 inches per hour. Now, lets also assume the rangeland is experiencing a rain, which falls at a rate of 1.5 inches per hour. This means that for each hour of rain, we have 1 inch of rain that is not able to infiltrate, and it moves down hill as runoff.
The actual infiltration capacity of a soil surface changes with percent soil moisture. That is, when a soil is initially dry, the rate at which it will absorb water is much greater. As the soil becomes saturated, its infiltration capacity drops, and eventually stabilizes. Figure 2 shows a typical infiltration curve. Note how the infiltration rate compares to the rainfall intensity as described in the previous example.
The two most important factors influencing the rate of infiltration are soil and vegetation. In order to infiltrate, water needs pathways. The soil properties which determine the size, shape, and conductivity of these pathways, are texture (sand, silt, clay), and aggregation. The first property can not be influenced, but the latter is strongly related to vegetation. Another effect that vegetation has on soil is providing the soil root zone with organic matter that increases its ability to hold water, thus reducing deep percolation. Vegetation also slows down water that is running off, allowing more time for it to infiltrate. The physical effects of root extension and the addition of organic matter stimulate the formation and stabilization of granular (crumb) aggregates. When this condition occurs we have well-aerated soils with more and larger pores (water pathways). In general, vegetative cover has more influence on infiltration rates than do the soil type and texture (Schwab, et al. 1993). Figure 3 shows cumulative infiltration over time for various surface conditions determined for three South Carolina soils.
A study was started in 1998 to quantify the relationship between grazing intensity and infiltration capacity of Missouri Coteau rangeland soils. For a duration of two growing seasons, infiltration data will be collected from the CGREC grazing treatments. These treatments have been under the influence of a specific grazing intensity for a period of 9 years. The treatments include no grazing, light, moderate, heavy, and extreme grazing.
A ring infiltrometer is used to measure infiltration rates. In this method, the cylinder (ring), is inserted into the ground and filled with water to a determined level. The rate at which the water falls inside the ring over a period of six hours is recorded. Simultaneously, changes in percent soil moisture are measured using TDR (Time Domain Reflectometry). Figure 4 shows the results of two infiltration tests; one from a moderately grazed site, and one from an extremely grazed site. As can be seen, there is almost a 1.5 inch /hour infiltration rate difference between these two test sites. The infiltration capacity of the moderately grazed site is stabilizing around 2 inches /hour, whereas the infiltration rate of the extremely grazed pasture is stabilizing at 0.5 inches /hour. Thus, for a storm event with a rainfall intensity of 2 inches /hour or less, the moderately grazed pasture will allow all the rainwater to infiltrate and will produce no runoff. The extremely grazed pasture will produce runoff for any storm falling at an intensity greater than 0.5 inches /hour. From these preliminary results it is clear that soils under differing grazing management schemes differ in their ability to conserve water. The curves shown in Figure 4 can also be fit to an equation that describes vertical infiltration under ponded conditions. In this way, the differences in infiltration characteristics over time can be quantified. A complete summary of the results will be published in the 1999 Grass & Beef Research Review.
The results of this study will indicate the degree to which soil infiltration rates are affected by grazing. Knowing these differences, and by looking at historical records of rainfall intensities, we can determine the best management practices for rangeland water conservation in central North Dakota.
Schwab, G.O., D.D. Fangmeier, W.J. Elliot, and R.K. Frevert. 1993. Soil and Water Conservation Engineering. John Wiley & Sons, Inc., New York, NY.
McGinty, A., T.L. Thurow, and C.A. Taylor. 1996. Improving Rainfall Effectiveness On Rangeland. http://texnat.tamu.edu/pubs/1-5029/1-5029-1.htm
Graduate Research Assistant
NDSU Department of Civil Engineering
North Dakota State University
Fargo, ND 58105