Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models

Statistics and Data Analysis in Geology

Human activity and related land use change are the primary cause of accelerated soil erosion, which has substantial implications for nutrient and carbon cycling, land productivity and in turn, worldwide socio-economic conditions. Here we present an unprecedentedly high resolution (250 × 250 m) global potential soil erosion model, using a combination of remote sensing, GIS modelling and census data. We challenge the previous annual soil erosion reference values as our estimate, of 35.9 Pg yr−1 of soil eroded in 2012, is at least two times lower. Moreover, we estimate the spatial and temporal effects of land use change between 2001 and 2012 and the potential offset of the global application of conservation practices. Our findings indicate a potential overall increase in global soil erosion driven by cropland expansion. The greatest increases are predicted to occur in Sub-Saharan Africa, South America and Southeast Asia. The least developed economies have been found to experience the highest estimates of soil erosion rates.

/pdf/an-assessment-of-the-global-impact-of-21st-century-land-use-rdg7ojdzqi.pdf

An assessment of the global impact of 21st century land use change on soil erosion.

Performance measures (PMs) and corresponding performance evaluation criteria (PEC) are important aspects of calibrating and validating hydrologic and water quality models and should be updated with advances in modeling science. We synthesized PMs and PEC from a previous special collection, performed a meta-analysis of performance data reported in recent peer-reviewed literature for three widely published watershed-scale models (SWAT, HSPF, WARMF), and one field-scale model (ADAPT), and provided guidelines for model performance evaluation. Based on the synthesis, meta-analysis, and personal modeling experiences, we recommend coefficient of determination (R2; in conjunction with gradient and intercept of the corresponding regression line), Nash Sutcliffe efficiency (NSE), index of agreement (d), root mean square error (RMSE; alongside the ratio of RMSE and standard deviation of measured data, RSR), percent bias (PBIAS), and several graphical PMs to evaluate model performance. We recommend that model performance can be judged satisfactory for flow simulations if monthly R2 0.70 and d 0.75 for field-scale models, and daily, monthly, or annual R2 0.60, NSE 0.50, and PBIAS ≤ ±15% for watershed-scale models. Model performance at the watershed scale can be evaluated as satisfactory if monthly R2 0.40 and NSE 0.45 and daily, monthly, or annual PBIAS ≤ ±20% for sediment; monthly R20.40 and NSE 0.35 and daily, monthly, or annual PBIAS ≤ ±30% for phosphorus (P); and monthly R2 0.30 and NSE 0.35 and daily, monthly, or annual PBIAS ≤ ±30% for nitrogen (N). For RSR, we recommend that previously published PEC be used as detailed in this article. We also recommend that these PEC be used primarily for the four models for which there were adequate data, and used only with caution for other models. These PEC can be adjusted within acceptable bounds based on additional considerations, such as quality and quantity of available measured data, spatial and temporal scales, and project scope and magnitude, and updated based on the framework presented herein. This initial meta-analysis sets the stage for more comprehensive meta-analysis to revise PEC as new PMs and more data become available.

Hydrologic and Water Quality Models: Performance Measures and Evaluation Criteria

Nutrient requirements of beef cattle

Cover crops alter many aspects of the hydrologic cycle. They increase evapotranspiration while growing and can enhance water infiltration into soil, slow runoff rates, and reduce soil erosion in both conventional-till and no-till systems throughout the year. However, the difference between the results of plot and watershed studies demonstrate that caution should be taken in extrapolating plot data to watershed scales. As scale increases, so does the influence of hydraulically-controlling subsurface soil horizons. Unfortunately, most of the available cover crop research comes from relatively small plats and very few watershed studies have been initiated in recent years. Perennial cover crops offer the potential for altering the porosity of subsurface soil horizons so as to increase future soil productivity and reduce future runoff amounts and rates.

Cover crop impacts on watershed hydrology

The majority of sediment leaving catchments may be from streambank failure. Seepage erosion of unconsolidated sand above a restrictive layer is an important erosion process in incised streams that leads to streambank failure by undercutting banks. The objective of this study was to determine the impact of soil properties on seepage erosion and the resulting streambank failure. Seepage flow and sediment concentrations were measured in situ at eight locations along the banks of a deeply incised stream in northern Mississippi. Using field observations as a guide, the soil profile conditions of a shallow (45 cm) streambank, consisting of 30 cm of topsoil, a 10 cm conductive layer, and a 5 cm restrictive layer, were mimicked in laboratory lysimeter experiments to quantify the hydrologic properties controlling seepage erosion and bank failure under a 40 cm head. The time to flow initiation and the flow rate were linearly related to the slope of the restrictive layer. Seepage erosion began within minutes of flow initiation and resulted in substantial (3 to 34 cm) undercutting of the bank. Sediment concentrations of seeps were as high as 660 g l−1in situ and 4500 g l−1 in the lysimeters. Sediment concentrations were related to the layer slope, thereby indicating the importance of detailed site characterization. The USDA-ARS Streambank Stability model demonstrated the increase in instability of banks due to undercutting by seepage erosion, but failed to account for the sediment loss due to sapping for stable banks and overestimated the sediment loads for failed banks. Published in 2006 by John Wiley & Sons, Ltd.

Soil properties controlling seepage erosion contributions to streambank failure

Universal Soil Loss Equation and Revised Universal Soil Loss Equation

Stiff-grass hedges can resist, retard, and disperse concentrated flows of runoff; trap suspended sediment; and reduce ephemeral gully development. Flume experiments were conducted at a 5% grade using several combinations of four grass species, four types of sediment, and eight flow rates ranging from 0.33 to 2.66 m3/min-m. Sediment trapping resulted primarily from deposition in the backwater upstream of the grass, rather than by filtration in the grass. Sediment was initially deposited 1 to 2 m downslope from a hydraulic jump transition that formed at the upper edge of the backwater. From this position, a delta of sediment mostly coarser than 125 mm grew back toward the hydraulic jump until flow depth became shallow enough that bedload transport was initiated and the delta began to advance toward the grass. In the flow zone across the delta, slope steepness approached 1 to 2% and the flow was near critical depth. A steady-state model was developed that describes sediment trapping in the backwater area as a settling process controlled by sediment characteristics, flow rate, and backwater depth at the grass. This model underpredicted trapping of fine sediment and overpredicted trapping of coarse sediment in situations where the delta neared the grass.

Depositional Patterns of Sediment Trapped by Grass Hedges

Cover-crop residues left on the soil surface as a mulch in no-till crop production systems protect the soil from erosion, increase water infiltration and suppress weeds. Because many growers using cover crops want to reduce chemical inputs, non-chemical methods of killing or suppressing cover crops are needed. In the first part of this paper we review the current literature and discuss advantages and disadvantages of five mechanical methods for killing cover crops, i.e., mowing, rolling, roll-chopping, undercutting and partial rototilling. We also report on three new studies that broaden the current literature, including planting into freshly killed residue. In the first study, the use of planter attachments to remove surface residues from the planter row improved stands when cotton was no-till planted 2-7 days after mowing cover crops in Mississippi. In the second study, 100% of a rye/vetch cover crop in Missouri was killed by mowing, and greater than 90% was killed by roll-chopping. Cotton stands were reduced by the use of row cleaners that clogged when the cover crop was roll-chopped or mowed on the same day that the crop was planted. The third study evaluated three methods of mechanically killing summer cover crops in North Carolina. Undercutting provided greater than 95% kill for five of six broadleaf species, and two of five grass species. Mowing effectively killed all six broadleaf cover crops, but re-growth occurred with three of five grasses, with the exception of nearly mature German foxtail millet and mature Japanese millet. In general, rolling did not effectively kill broadleaf or grass cover crops, with the exception of nearly mature German foxtail millet, mature Japanese millet and mature buckwheat.

Seth M. Dabney

Papers

Cover crop impacts on watershed hydrology

Soil properties controlling seepage erosion contributions to streambank failure

Universal Soil Loss Equation and Revised Universal Soil Loss Equation

Depositional Patterns of Sediment Trapped by Grass Hedges

Killing cover crops mechanically: Review of recent literature and assessment of new research results