Research Agricultural Engineer
Mark A. Nearing
Mark Nearing has a Ph.D. in Civil Engineering (1986) and an M.S. in Soil Science (1984) from Purdue University. He has worked as a scientist for the Agricultural Research Service since 1986. Most of Dr. Nearing's work has been in the area of soil erosion and conservation, including the understanding of basic erosion processes, field measurement of erosion rates, and the development and testing of soil erosion models. Recent work focuses on erosion in semi-arid rangeland environments.
Soil is critical to the long-term sustainability of semiarid rangelands. Although research to understand the factors affecting erosion, as well as the on-site and off-site impacts of erosion, have advanced our ability to sustainably manage land, technologies for quantifying and predicting soil erosion on rangelands have not kept pace with applications for agricultural cropland. Changing demographics and population increases in semiarid areas around the world have heightened the awareness of land use impacts and the need for technologies to quantify and predict the impacts of land management on soil erosion. The need for prediction technology is paralleled by the need for decision tools and information delivery mechanisms. We are working to improve the scientific understanding of hillslope and watershed scale erosion, develop new technologies for measuring and monitoring erosion and sediment transport, improve conservation practices on rangelands, improve prediction technologies, and improve technologies for decision making.
Recent work includes: 1) modeling soil erosion at the hillslope scale in western rangelands; 2) using radioactive isotopes to quantify historic rates of soil erosion on rangelands; and 3) documenting the potential effects of future climate change on soil erosion and conservation.
Rangeland Soil Erosion Modeling
Runoff and erosion rates predicted from models for rangelands are important quantitative indicators for rangeland health and for assessing the effectiveness of conservation practices. Government agencies, rangeland managers, conservationists and rangeland scientists need technology that will allow them to estimate these rates. In recent work we developed a new technology for predicting infiltration, runoff, and soil erosion specifically for rangelands of the western United States. The Rangeland Hydrology and Erosion Model (RHEM) is scientifically rigorous, in that it is represents state-of-the-art understanding of infiltration, runoff, and soil erosion processes on rangelands. It is based on an extensive set of measured data that has been collected over the past 20 years. Also, it accessible to the average user via the internet, and requires only information that is commonly collected by or available to rangeland scientists and managers. This new technology will enable improved estimation of hydrology and erosion by water on rangelands across the western United States, which will lead to an improved ability to manage this extensive and sometimes fragile natural resource.
Using Isotopes to Quantify Historic Soil Erosion Rates
Quantitative data on hillslope-scale soil erosion rates in arid and semiarid rangelands are very limited. Erosion stations were established across much on the United States beginning in the 1930s for the purpose of quantifying soil erosion rates, primarily in row-cropped agricultural lands. These data formed the basis for the implementation of soil conservation programs in the United States over the past decades. The lack of similar basic erosion rate data for western rangelands has hampered a similar application of erosion prediction tools for implementing conservation plans in much of the West, which instead rely more on qualitative assessments of rangeland condition. In addition, data are needed for understanding the relationships between soil erosion and states within rangeland ecological sites.137Cs is an artificial radionuclide with a half-life of approximately 30 years. It was globally distributed by atmospheric tests of atomic weapons largely from the mid 1950s through 1963. The measurement of 137Cs content in soils has been extensively used to study soil erosion and sediment redistribution in agricultural lands. The 137Cs technique is based on the fact that once 137Cs reaches the soil, it is strongly and rapidly adsorbed on finer soil and organic particles, and it is essentially non-exchangeable in soils. Erosion and deposition rates are calculated by conversion models with the information on the gain or loss of 137Cs inventories at a sampling point, relative to a reference inventory, which is the fallout input to an undisturbed watershed. Thus, the erosion and deposition rates obtained by 137Cs measurements are time-integrated, medium-term (~50 years) average rates of soil redistribution. A great advantage of this technique is the ability to provide spatially-distributed information on soil redistribution without long-term monitoring.
Climate Change and Soil Erosion
Climate change presents a major challenge to sustainable land management. The effects of climate change on soil and water resources on agricultural land are critical and should be addressed with U.S. conservation policy and practice. The magnitudes and extent of increased rates of soil erosion and runoff that could occur under simulated future precipitation regimes are large, and analyses of the climate record in the United States have shown that changes in precipitation have already occurred across the country, with large observed trends in precipitation and the bias toward more extreme precipitation events. Increases in soil erosion and runoff from cropland has the potential to reverse much of the past progress that has been made in reducing soil degradation and water pollution from cropland in the United States. In the arid and semi-arid rangelands of the western U.S. there is concern that increased periods of drought accompanying a drier environment can change ecosystem composition and reduce vegetative cover that protects the soil from both wind and water erosion. Soil degradation is closely related to soil erosion, and these factors have the potential to impact soil quality, productivity, and associated ecosystem services.
Synergistic opportunities for combating the effects of climate change on soil degradation are available to us. For example, reducing erosion through conservation practices will improve soil quality, land sustainability and crop production, and it can also act to sequester carbon in the soil rather than having it released into the atmosphere. Another example is the use of improved-efficiency irrigation practices, which not only will make more effective use of increasingly stressed future water supplies, but also reduce soil erosion. Also, temporal updates to conservation planning and assessment tools (e.g., in terms of currently changing rainfall erosivity and management practices) have the potential to raise awareness of producers and land managers to employ adaptive management, targeted conservation, and improved conservation practices in general. On the other hand, there is also a potential for negative indirect feedbacks of climate change on the soil resource that need to be recognized. One example is the production of annual crops for biofuel production, which involves removal of crop residue necessary to protect the soil surface from erosion. Another example is a change in crops produced as climate shifts, such as the potential for increased production of soybeans, which are more susceptible to causing erosion, over maize in parts of the Midwest. Even though climate change has the potential to significantly impact the soil resource, in many cases to the negative, it is clear that proper land use and management with the conservation tools we currently possess can protect and sustain the soil resource under a changing climate within a shifting production and management environment. In other words, we have many of the tools needed to address the problems that are arising; what we need is policy and awareness to see that they are addressed.