In the last iGrow article I wrote, I discussed soil health and the biotic state. In this article in the Rangeland Soil Health Article Series, I want to focus on the water cycle. How would you know if a pasture is showing signs of an inefficient water cycle? Indicators to evaluate the water cycle include gullies, blowouts, pedestaling, water flow patterns, and amount of litter (Figure 1). Water infiltration is mainly affected by soil texture (size of soil particles; sand, silt, or clay), soil structure (arrangement of soil particles), slope, and vegetation.
Figure 1. Color coded monitoring indicators of nutrient cycling, water cycling, the biotic state, and energy flow that are involved in ecosystem processes
(modified after Pyke et al. 2002; Pellant et al. 2005; Orchard 2013).
As managers, we can only control the type and vigor of the vegetation that grows on the land. Vegetation directly impacts the contribution of organic matter (roots and root exudates), which affects microbial activity and soil binding (soil aggregation from microbial gums). Vegetation and how intensely it is grazed also impacts the amount and type of litter on the surface. Rangeland, either sandy or clay soil types, have lower water infiltration rates from pastures managed with less litter and comprised of shorter species than pastures with more litter and taller species.
Grazing Levels & Sediment Runoff
In the 1960s, SDSU with help from the Soil Conservation Service set up experimental watersheds on the stocking rate study at the Cottonwood Range and Livestock Research Station. This study measured water and sediment runoff from heavily, moderately, and lightly grazed mixed-grass prairie rangeland. Runoff from the heavily grazed pasture was twice that of the lightly grazed pasture. Water lost as runoff causes gulley erosion and was identified as the main culprit of sediment flow from the Bad River Watershed into the Missouri River (Figure 2; read the whole article for more information).
Figure 2. Sediment from the Bad River entering into Lake Sharpe at Ft. Pierre.
Photo by Kurt Reitsma.
The other side of the water cycle occurs when there is not enough rain. Recently, SDSU researchers measured soil moisture status of the heavily, moderately, and lightly grazed pastures at Cottonwood during the drought of 2012. The heavily grazed pasture dried out about two weeks earlier than the moderately or lightly grazed pasture after major rainfall events (Figure 3).
Figure 3. Soil moisture at 3 inch depth measured during the grazing season of 2012 at the Cottonwood Range and Livestock Research Station in heavily (red line), moderately (green line), and lightly (blue line) grazed pastures.
(Smart et al. Unpublished data).
Changing the amount of litter is probably the most effective management strategy to improve the water cycle. Healing gullies and blowouts using a combination of mechanical renovation and seeding, or feeding livestock hay and allowing them to trample organic matter into the soil will only temporarily fix the problem if the proper amount of litter and residual vegetation is not left at the end of the grazing season. Setting the correct stocking rate and providing seasonal deferments or an entire year rest will provide vegetation that can be trampled by livestock or knocked down by snow to add litter to the soil surface. Letting vegetation grow tall and mature followed by high stock density grazing is a very useful way to speed up this process. Producers must be careful to monitor animal performance while using this technique because the forage will be mature and low in forage quality. Long-term management toward more diverse mid- to tallgrass species is the best strategy to ensure an effective water cycle. Rangelands that have mid- to tallgrass species produce more biomass and litter and have higher infiltration rates.
Visit the links below to view other articles in the Rangeland Soil Health series: