June E. Wolfe

MODELING THE EFFECTIVENESS OF CONSERVATION PRACTICES AT SHOAL CREEK WATERSHED, TEXAS, USING APEX

This study was conducted to evaluate the performance of the Agricultural Policy/Environmental eXtender (APEX) model using daily storm event runoff and sediment yields (1997-2005) collected at the outlet of the 22.5 km2 Shoal Creek watershed. This watershed only has intermittent streams. The watershed is within the U.S. Armys Fort Hood military reservation in central Texas. It received a combination of erosion control practices including implementation of gully plugs and contour soil ripping. APEX was calibrated and validated with a 183-subarea configuration delineated from a 5 m digital elevation model. Results from model calibration and validation confirmed that APEX was able to realistically estimate daily runoff and sediment yield for both the pre- and post-BMP conditions, as evidenced by R2 values ranging from 0.60 to 0.80 and Nash-Sutcliffe efficiency (EF) values ranging from 0.58 to 0.77 with an exception of 0.33. During the post-BMP period, the total sediment yield was significantly less than that from the pre-BMP period, even though the corresponding total precipitation amount from the post-BMP events (1025 mm) was more than that from the pre-BMP events (668 mm). The simulated sediment yield was summed up to 24.3 Mg ha-1 for the pre-BMP events and 7.6 Mg ha-1 for the post-BMP events, which were very close to the measured values of 24.9 and 8.1 Mg ha-1, respectively. The benefits of the existing BMPs were quantified as a reduction of 52% in runoff and 86% in sediment yield based on comparisons between predictions from the run using the pre-BMP model setup and measured values under BMP conditions. The results suggest that APEX is capable of simulating conservation practices on military landscapes, and that it a useful tool for scenario analysis to evaluate the effectiveness of conservation practices.

Conservation Effects Assessment Project research in the Leon River and Riesel watersheds

The Leon River basin was selected as a benchmark watershed for the Conservation Effects Assessment Project to complement the historical USDA Agricultural Research Service experimental watersheds near Riesel, Texas. Excessive nutrient and bacteria concentrations contributed by agricultural, urban, and natural sources are the primary water quality concerns. Modeling and field evaluations of the hydrologic impact and soil and water quality response to tillage and nutrient management practices are the primary research themes of this project. Water quality data from 15 Leon River watersheds (0.3 ha [0.75 ac] to 6,070 km2 [2,340 mi2]) and 13 Riesel watersheds (1.2 ha [3.0 ac] to 70.4 ha [174 ac]) has improved modeling of phosphorus transformation and transport routines. Modeling research also coupled field- and farm-scale model output to improve the basin-scale Soil and Water Assessment Tool (SWAT) for the national assessment of conservation practices. Additional key products of Conservation Effects Assessment Project research include innovative erosion control methods on military lands, enhanced carbon sequestration estimates for various agricultural land uses, and improved understanding of environmental and economic impacts of organic fertilizer application.

Evaluation of removal of orthophosphate and ammonia from rainfall runoff using aboveground permeable reactive barrier composed of limestone and zeolite

This paper evaluates the design and performance of an Aboveground Permeable Reactive Barrier (APRB) system made of polyethylene mesh bags (FlowBags) containing crushed limestone and zeolite for adsorption of orthophosphate-P (PO4-P) and ammonia-N (NH4-N) from rainfall runoff. Laboratory batch experiments, simulated runoff experiments and actual APRB implementations were performed to evaluate the performance of the APRB. Batch experiments were performed to determine adsorption efficiency of crushed zeolite and limestone as reactive materials in APRB for removal of dissolved ammonium nitrogen and orthophosphate phosphorus from aqueous solutions under controlled laboratory conditions. Adsorption efficiencies of zeolite and limestone were tested individually and in combination. Results show adsorption efficiency increases when the materials are used in combination. Effects of particle size, contact time, pH, and temperature were studied. Major emphasis was given to short contact times because the contact of rainfall runoff water under field conditions with APRBs would be ∼5 minutes. Maximum removal of ∼70% PO4-P and NH4-N was seen at 45°C in 5 minutes within a pH range of 8–11. Optimum adsorbent concentration was 0.3 ppm with 20 g limestone and 10 g of zeolites. Simulated field experiments and actual APRB field installations showed variable results. Results from field evaluations of APRB showed mixed results from very high to negligible removal of orthophosphate-P and ammonia-N at different monitoring sites and storm events. Such variability may be due to the design of the bags, other biotic and abiotic factors and various physical factors, which are absent in the laboratory conditions. Some APRB design problems were also observed under field conditions and solutions are suggested. Overall results indicate that APRBs composed of combinations of crushed zeolite and limestone will offer an effective low maintenance and green alternative to remove dissolved nutrients from runoff and protect surface water resources from eutrophication.

Characterizing a Shallow Groundwater System beneath Irrigated Sugarcane with Electrical Resistivity and Radon (222Rn), Puunene, Hawaii

ABSTRACT In this study, we use a combination of electrical resistivity profiling and radon (222Rn) measurements to characterize a shallow groundwater system beneath the last remaining, large-scale sugarcane plantation on Maui, Hawaii. Hawaiian Commercial & Sugar Company has continuously operated a sugarcane plantation on the western flank of Haleakala Volcano since 1878. The sugarcane is irrigated with a combination of surface water brought through tunnels from the wetter, eastern flank of Haleakala Volcano and groundwater from wells within the plantation. To assess the flow of irrigation water through the shallow subsurface, we collected a representative topo-sequence of four 2-D resistivity profiles that sample different topographic and hydrologic settings within the plantation. The profiles show a down-slope-thickening (0 to 20 m), surficial low-resistivity (10-200 Ohm-m) layer extending from the upslope limit of irrigated sugarcane to the lowest elevations of the plantation. At a canal crossing, the l...

Contour ripping is more beneficial than composted manure for restoring degraded rangelands in Central Texas

Rangelands in the United States that have been the site of military training exercises have suffered extensive ecological damage, largely because of soil compaction, creation of ruts, and damage to or destruction of vegetation--all of which lead to higher runoff and accelerated erosion. In this paper we report on a study carried out within the Fort Hood Military Reservation in Central Texas, where we evaluated the extent to which application of composted dairy manure and contour ripping affect soil infiltrability, amount of runoff, and nutrient concentrations in runoff. We conducted experiments at two locations, using rainfall simulation at one and monitoring discharge from small (0.3-ha) watersheds at the other. At the rainfall simulation site, we used six levels of compost application: 4, 8, 12, 16, 20, and 24 Mg/ha. We found that compost application had little effect on runoff, soil infiltration, sediment production, or nutrient concentrations in the runoff--except at the micro-watershed scale (12 and 24 Mg/ha); in this case, nutrient concentrations in runoff were initially high (for the rainfall simulations done immediately after compost application). In contrast, contour ripping--carried out 22 months after compost application on two of the micro-watersheds--was highly effective: runoff on the treated micro-watershed was reduced by half compared with the untreated micro-watershed. Our results suggest that (1) one-time applications of composted dairy manure do little to enhance infiltration of degraded rangelands over the short term (at the same time, these experiments demonstrated that compost application poses very little risk to water quality); and (2) for degraded rangelands with limited infiltration capacity, contour ripping is an effective strategy for increasing infiltration rates.

Technical Note: A Constant-Volume Artificial Stream for Reducing Variation in Aquatic Process Measurements

This article describes the construction, operation, and assessment of a constant-volume artificial stream designed to control water quality and reduce variation to improve aquatic process measurements. The system was used to investigate suspended clay influence on phosphorus acquisition (load buffering) by lotic periphyton. Spatial and temporal heterogeneity represent the greatest sources of variation in the distribution and activity of aquatic biota in natural systems. Imposing experimental treatments simultaneously addresses temporal variation. Spatial variability is managed by controlling physical and chemical variables such as temperature, light, substrate, flow, and water quality. Controlling environmental conditions leads to reduced variation in biotic state, which improves the measurement of biotic processes. This artificial stream design minimizes water volume fluctuation, which facilitates solution manipulation and measurement and reduces environmental variation. A unique reservoir and valve arrangement compensates for evaporative water loss, allowing the operating water volume of individual stream units to be maintained (< ±1%) over long periods. In pre-experiment assessments, mean total dissolved salts were maintained to within 3% of their initial concentration over a one-week period, and suspended clay turbidity was within 4.3% after some initial settling. During experimental use, artificial stream variation in periphyton biomass and phosphorus uptake was less than that reported from several natural streams.

Land Degradation in Rangeland Ecosystems

Rangelands provide an array of ecosystem services such as food, fiber, water, recreation, minerals, and are important to the livelihoods of people across the globe, especially in developing countries. Competing land uses, overgrazing, extreme climate events, and socioeconomic changes are resulting in rangeland degradation in many parts of the world. Given our reliance on rangelands, degradation of this resource can have far-reaching effects. In this chapter, causes of rangeland degradation are examined. Indicators that can be used to identify degradation and methods for assessing the degree of degradation in rangeland ecosystems are discussed. Options and considerations for restoring disturbed rangeland are presented, in addition to future directions in rangeland degradation monitoring and assessment.

Simulated and Measured Sediment Loadings Before and After BMP Implementation on a Military Base Watershed

Erosion from military training areas is becoming an increasing problem that can contribute significant amounts of sediment loadings into nearby streams. For example, preliminary results from a heaviliy traveled 22 km2 Shoal Creek military base watershed in central Texas indicated as much as 27,913 Tons of sediment was lost in 2000. To help reduce erosion, NRCS personnel have implemented BMPs, such as subsoil ripping and installation of rock structures in eroded channels in Shoal Creek. To assist in evaluating the long-term impacts of implementing such practices, the SWAT (Soil & Water Assessment Tool) model compared estimated runoff and sediment loadings at the outlet of the watershed before and after implementation of the management practices.