Mary H. Nichols

Measured Sediment Yield Rates From Semiarid Rangeland Watersheds

Abstract Data describing long-term sediment yield rates on semiarid rangeland watersheds are relatively rare. To augment existing data and gain a better understanding of the controlling variables, sediment yields from 8 subwatersheds within the US Department of Agriculture–Agricultural Research Service Walnut Gulch Experimental Watershed in southeastern Arizona were computed from stock pond sediment accumulation measurements, water level records, and estimates of sediment transported in pond overflows. Sediment accumulation records ranging from 30 to 47 years were evaluated for subwatersheds ranging in size from 35.2 to 159.5 ha. Sediment yield ranged from 0.5 to 3.0 m3·ha−1·y−1, with a mean of 1.4 m3·ha−1·y−1 and a standard deviation of 1.0 m3·ha−1·y−1. As expected, runoff volume was a significant factor (P = 0.005) in explaining the variability in sediment yield, but regression analysis demonstrated other variables are important. For example, the ratio of watershed area to main channel length significantly described (P = 0.06) sediment yield, suggesting that more detailed measurements are needed to characterize channel networks to relate internal watershed sediment transport and deposition processes to sediment delivery at the outlets. To generalize sediment yield rates across rangeland regions, additional research is necessary to determine the relative influence of rainfall and runoff patterns, and watershed physiographic and geomorphic characteristics on sediment transport.

Effect of check dams on runoff, sediment yield, and retention on small semiarid watersheds

Erosion dynamics in semiarid environments is defined by high magnitude, low frequency rainfalls that produce runoff with high sediment concentration. Check dams are widely used in this environment as a sedimentation mitigation technique, however their impact on overall watershed sediment balance is not well known. In 2008 a total of 37 loose rock semipermeable check dams were installed on two small (4 and 3.1 ha [9.8 and 7.6 ac]) watersheds located on the alluvial fan of the Santa Rita Mountains in southern Arizona. Each watershed was equipped with a rain gauge, supercritical flow flume, and sediment sampler. The runoff and sediment yield characteristics following the check dam installation were compared with 35 years of historical records. Impacts of the check dams on runoff from major rainstorms were not detectable; however the number of runoff events generated by small (less than one year recurrence interval) rainstorms decreased by 60%. During four years check dams retained 75 t (82.6 tn) of sediment (50% of sediment yield) and were filled to more than 80% of their capacity. Depositional areas upstream of the dams have potential to support watershed restoration.

Geographic information systems database, Walnut Gulch Experimental Watershed, Arizona, United States

The geographic information systems (GIS) database complementing the Walnut Gulch Experimental Watershed (WGEW) special section papers in this issue of Water Resources Research is described. Spatial data layers discussed here will be especially useful to modelers interested in simulating the spatial and temporal characteristics of rainfall, runoff, erosion, and sedimentation processes on the WGEW. All data are available as either images or individual GIS data layers (vector or raster format) via the U.S. Department of Agriculture Agricultural Research Service Southwest Watershed Research Center at http://www.tucson.ars.ag.gov/dap/. Standard metadata are provided with attending projection information and restrictions on use.

Time series analyses of global change data

The hypothesis that statistical analyses of historical time series data can be used to separate the influences of natural variations from anthropogenic sources on global climate change is tested. Point, regional, national, and global temperature data are analyzed. Trend analyses for the period 1901-1987 suggest mean annual temperatures increased (in degrees C per century) globally at the rate of about 0.5, in the USA at about 0.3, in the south-western USA desert region at about 1.2, and at the Walnut Gulch Experimental Watershed in south-eastern Arizona at about 0.8. However, the rates of temperature change are not constant but vary within the 87-year period. Serial correlation and spectral density analysis of the temperature time series showed weak periodicities at various frequencies. The only common periodicity among the temperature series is an apparent cycle of about 43 years. The temperature time series were correlated with the Wolf sunspot index, atmospheric CO(2) concentrations interpolated from the Siple ice core data, and atmospheric CO(2) concentration data from Mauna Loa measurements. Correlation analysis of temperature data with concurrent data on atmospheric CO(2) concentrations and the Wolf sunspot index support previously reported significant correlation over the 1901-1987 period. Correlation analysis between temperature, atmospheric CO(2) concentration, and the Wolf sunspot index for the shorter period, 1958-1987, when continuous Mauna Loa CO(2) data are available, suggest significant correlation between global warming and atmospheric CO(2) concentrations but no significant correlation between global warming and the Wolf sunspot index. This may be because the Wolf sunspot index apparently increased from 1901 until about 1960 and then decreased thereafter, while global warming apparently continued to increase through 1987. Correlation of sunspot activity with global warming may be spurious but additional analyses are required to test this hypothesis. Given the inconclusive correlation between temperature and solar activity, the significant intercorrelation between time, temperature, and atmospheric CO(2) concentrations, and the suggestion of weak periodicity in the temperature data, additional research is needed to separate the anthropogenic component from the natural variability in temperature when assessing local, regional, and global warming trends.

Runoff and erosional responses to a drought‐induced shift in a desert grassland community composition

In contrast, measurements on small runoff plots on the hillslopes of the same watershed showed a significant increase in sediment discharge that continued after E. lehmanniana replaced native grasses. Together, these findings suggest alteration in plant community increased sediment yield but that hydrological responses to this event differ at watershed and plot scales, highlighting the geomorphological controls at the watershed scale that determine sediment transport efficiency and storage. Resolving these scalar issues will help identify critical landform features needed to preserve watershed integrity under changing climate conditions.

Incorporating Hydrologic Data and Ecohydrologic Relationships into Ecological Site Descriptions

ABSTRACT The purpose of this paper is to recommend a framework and methodology for incorporating hydrologic data and ecohydrologic relationships in Ecological Site Descriptions (ESDs) and thereby enhance the utility of ESDs for assessing rangelands and guiding resilience-based management strategies. Resilience-based strategies assess and manage ecological state dynamics that affect state vulnerability and, therefore, provide opportunities to adapt management. Many rangelands are spatially heterogeneous or sparsely vegetated where the vegetation structure strongly influences infiltration and soil retention. Infiltration and soil retention further influence soil water recharge, nutrient availability, and overall plant productivity. These key ecohydrologic relationships govern the ecologie resilience of the various states and community phases on many rangeland ecological sites (ESs) and are strongly affected by management practices, land use, and disturbances. However, ecohydrologic data and relationships are often missing in ESDs and state-and-transition models (STMs). To address this void, we used literature to determine the data required for inclusion of key ecohydrologic feedbacks into ESDs, developed a framework and methodology for data integration within the current ESD structure, and applied the framework to a select ES for demonstrative purposes. We also evaluated the utility of the Rangeland Hydrology and Erosion Model (RHEM) for assessment and enhancement of ESDs based in part on hydrologic function. We present the framework as a broadly applicable methodology for integrating ecohydrologic relationships and feedbacks into ESDs and resilience-based management strategies. Our proposed framework increases the utility of ESDs to assess rangelands, target conservation and restoration practices, and predict ecosystem responses to management. The integration of RHEM technology and our suggested framework on ecohydrologic relations expands the ecological foundation of the overall ESD concept for rangeland management and is well aligned with resilience-based, adaptive management of US rangelands. The proposed enhancement of ESDs will improve communication between private land owners and resource managers and researchers across multiple disciplines in the field of rangeland management.

A Simulation Model for Erosion and Sediment Yield at the Hillslope Scale

As a physical feature of the landscape, hillslopes connect high points with low points. A hillslope can be defined as the zone of the landscape from the crest of a ridge along the slope in the direction of flow to a defined drainage, water body, or other feature that interrupts the overland flow profile at the toe of the slope. The evolution and visible forms of hillslopes are in large part determined by the effects of water driven erosion. In the absence of activities such as land forming, grading, cultivation, etc., hillslopes are relatively stable and their forms evolve slowly.

Very-High-Resolution Panoramic Photography to Improve Conventional Rangeland Monitoring

Abstract Rangeland monitoring often includes repeat photographs as a basis for documentation. Whereas photographic equipment and electronics have been evolving rapidly, photographic monitoring methods for rangelands have changed little over time because each picture is a compromise between resolution and area covered. Advances in image sensors, storage media, and image-processing software allow enormous amounts of information to be collected efficiently and inexpensively, so multiple pictures taken at full zoom can be combined into a single high-resolution panoramic image. This project was initiated to integrate very-high-resolution panoramic images with conventional rangeland monitoring methods addressing three resource management categories: riparian areas, wildlife, and invasive species.

The Rangeland Hydrology and Erosion Model: A Dynamic Approach for Predicting Soil Loss on Rangelands

In this study, we present the improved Rangeland Hydrology and Erosion Model (RHEM V2.3), a process-based erosion prediction tool specific for rangeland application. The article provides the mathematical formulation of the model and parameter estimation equations. Model performance is assessed against data collected from 23 runoff and sediment events in a shrub-dominated semiarid watershed in Arizona, USA. To evaluate the model, two sets of primary model parameters were determined using the RHEM V2.3 and RHEM V1.0 parameter estimation equations. Testing of the parameters indicated that RHEM V2.3 parameter estimation equations provided a 76% improvement over RHEM V1.0 parameter estimation equations. Second, the RHEM V2.3 model was calibrated to measurements from the watershed. The parameters estimated by the new equations were within the lowest and highest values of the calibrated parameter set. These results suggest that the new parameter estimation equations can be applied for this environment to predict sediment yield at the hillslope scale. Furthermore, we also applied the RHEM V2.3 to demonstrate the response of the model as a function of foliar cover and ground cover for 124 data points across Arizona and New Mexico. The dependence of average sediment yield on surface ground cover was moderately stronger than that on foliar cover. These results demonstrate that RHEM V2.3 predicts runoff volume, peak runoff, and sediment yield with sufficient accuracy for broad application to assess and manage rangeland systems.

Correction to “Sediment yields from unit‐source semiarid watersheds at Walnut Gulch”

[1] This study reports sediment yields from seven small (0.18–5.42 ha) watersheds in Southern Arizona measured from 1995 to 2005. Sediment concentrations and total event sediment yields were related to storm-runoff characteristics, and statistical relationships were developed to estimate sediment yields for events with missing data. Precipitation ranged from 263 to 298 mm yr , runoff ranged from 8.2 to 26.4 mm yr , and sediment yields ranged from 0.07 to 5.7 t ha 1 yr , with an areal average of 2.2 t ha 1 yr . For six of the seven watersheds, between 6 and 10 events produced 50% of the total sediment yields over the 11-year period. On the seventh watershed, two storms produced 66% of the sediment because of differences in the geomorphology and vegetation characteristics of that area. Differences between sediment yields from all watersheds were attributable to instrumentation, watershed morphology, degree of channel incision, and vegetation.