Michele L. Reba

Comparing Simulated and Measured Sensible and Latent Heat Fluxes over Snow under a Pine Canopy to Improve an Energy Balance Snowmelt Model

Abstract During the second year of the NASA Cold Land Processes Experiment (CLPX), an eddy covariance (EC) system was deployed at the Local Scale Observation Site (LSOS) from mid-February to June 2003. The EC system was located beneath a uniform pine canopy, where the trees are regularly spaced and are of similar age and height. In an effort to evaluate the turbulent flux calculations of an energy balance snowmelt model (SNOBAL), modeled and EC-measured sensible and latent heat fluxes between the snow cover and the atmosphere during this period are presented and compared. Turbulent fluxes comprise a large component of the snow cover energy balance in the premelt and ripening period (March–early May) and therefore control the internal energy content of the snow cover as melt accelerates in late spring. Simulated snow cover depth closely matched measured values (RMS difference 8.3 cm; Nash–Sutcliff model efficiency 0.90), whereas simulated snow cover mass closely matched the few measured values taken during...

Measurement of Surface Energy Fluxes from Two Rangeland Sites and Comparison with a Multilayer Canopy Model

Rangelands are often characterized by a patchy mosaic of vegetation types, making measurement and modeling of surface energy fluxes particularly challenging. The purpose of this study was to evaluate surface energy fluxes measured using three eddy covariance systems above and within two rangeland vegetation sites and use the data to improve simulations of turbulent energy fluxes in a multilayer plant canopy model: the Simultaneous Heat and Water (SHAW) model. Model modifications included adjustment of the wind profile roughness parameters for sparse canopies, extending the currently used K-theory approach to include influence of the roughness sublayer and stability functions within the canopy, and in a separate version of the model, introducing Lagrangian far-field turbulent transfer equations (L theory) in lieu of the K-theory approach. There was relatively little difference in simulated energy fluxes for the aspen canopy using L-theory versus K-theory turbulent transfer equations, but L theory tracked canopy air temperature profiles better during the growing season. Upward sensible heat flux was observed above aspen trees, within the aspen understory, and above sagebrush throughout the active snowmelt season. Model simulations confirmed the observed upward sensible flux during snowmelt was due to solar heating of the aspen limbs and sagebrush. Thus, the eddy covariance (EC) systems were unable to properly quantify fluxes at the snow surface when vegetation was present. Good agreement between measured and modeled energy fluxes suggest that they can be measured and simulated reliably in these complex environments, but care must be used in the interpretation of the results.

A statewide network for monitoring agricultural water quality and water quantity in Arkansas

The world population reached seven billion in 2011, and global population of nine billion is expected by 2050. To sustain agricultural production of food, fiber, feed, and fuel for the world population, agriculture requires water and nutrient inputs, which can impair water resources by decreasing water quality and availability. Both are concerns in the agricultural region of the Lower Mississippi River Basin (LMRB) and specifically in the state of Arkansas, where production of rice, cotton, soybean, and poultry are critical to the states economy. Water quality issues are related to excess nutrients running off of fields that subsequently influence local and regional water bodies (Carpenter et al. 1998). Water quantity issues are related to declines in groundwater caused by withdrawal rates that are greater than recharge rates. Conservation practices targeted at improving water resources and promoted through the Mississippi River Basin Healthy Watersheds Initiative (MRBI) are supported by USDA Natural Resources Conservation Service (USDA NRCS) and include a component dedicated to monitoring the water resources impact of these practices. A statewide monitoring network designed to collect water quality and water quantity data was established in 2010 in Arkansas. The network is made up of approximately 30 monitoring sites on 12…

Water quality of surface runoff and lint yield in cotton under furrow irrigation in Northeast Arkansas

Use of furrow irrigation in row crop production is a common practice through much of the Midsouth US and yet, nutrients can be transported off-site through surface runoff. A field study with cotton (Gossypium hirsutum, L.) was conducted to understand the impact of furrow tillage practices and nitrogen (N) fertilizer placement on characteristics of runoff water quality during the growing season. The experiment was designed as a randomized complete block design with conventional (CT) and conservation furrow tillage (FT) in combination with either urea (URN) broadcast or 32% urea ammonium nitrate (UAN) injected, each applied at 101kgNha-1. Concentrations of ammonium (NH4-N), nitrate (NO3-N), nitrite (NO2-N), and dissolved phosphorus (P) in irrigation runoff water and lint yields were measured in all treatments. The intensity and chemical form of nutrient losses were primarily controlled by water runoff volume and agronomic practice. Across tillage and fertilizer N treatments, median N concentrations in the runoff were <0.3mgNL-1, with NO3-N being relatively the highest among N forms. Concentrations of runoff dissolved P were <0.05mgPL-1 and were affected by volume of runoff water. Water pH, specific electrical conductivity, alkalinity and hardness were within levels that common to local irrigation water and less likely to impair pollution in waterways. Lint yields averaged 1111kgha-1 and were higher (P-value=0.03) in FT compared to CT treatments. Runoff volumes across irrigation events were greater (P-value=0.02) in CT than FT treatments, which increased NO3-N mass loads in CT treatments (394gNO3-Nha-1season-1). Nitrate-N concentrations in CT treatments were still low and pose little threat to N contaminations in waterways. The findings support the adoption of conservation practices for furrow tillage and N fertilizer placement that can reduce nutrient runoff losses in furrow irrigation systems.

Evaporation and the subcanopy energy environment in a flooded forest

Abstract The combination of tree canopy cover and a free water surface makes the subcanopy environment of flooded forested wetlands unlike other aquatic or terrestrial systems. Subcanopy vapour fluxes and energy budgets represent key controls on water level and understorey climate but are not well understood. In a permanently flooded forest in south‐eastern Louisiana, USA, an energy balance approach was used to address (a) whether evaporation from floodwater under a forest canopy is solely energy limited and (b) how energy availability was modulated by radiation and changes in floodwater heat storage. A 5‐month continuous measurement period (June–November) was used to sample across seasonal changes in canopy activity and temperature regimes. Over this period, the subcanopy airspace was humid, maintaining saturation vapour pressure for 28% of the total record. High humidity coupled with the thermal inertia of surface water altered both seasonal and diel energy exchanges, including atypical phenomena such as frequent day‐time vapour pressure gradients towards the water surface. Throughout the study period, nearly all available energy was partitioned to evaporation, with minimal sensible heat exchange. Monthly mean evaporation ranged from 0.7 to 1.7 mm/day, peaking in fall when canopy senescence allowed greater radiation transmission; contemporaneous seasonal temperature shifts and a net release of stored heat from the surface water resulted in energy availability exceeding net radiation by 30% in October and November. Relatively stable energy partitioning matches Priestley–Taylor assumptions for a general model of evaporation in this ecosystem.

Impact of cover crop and season on nutrients and sediment in runoff water measured at the edge of fields in the Mississippi Delta of Arkansas

Improved understanding of water quality at the edge-of-field (EOF) from production-size fields is needed to better inform agriculture and resource managers regarding sustainable farming practices and environmental stewardship. We measured runoff water quality at EOF of paired commercial fields in Mississippi and Craighead counties (total of four fields) in the Mississippi Delta region of eastern Arkansas, an understudied agricultural region. The paired fields had similar size, soil properties, and crop (cotton [Gossypium hirsutum L.] or corn [Zea mays L.]). Runoff water quality (nitrate [NO3], nitrite [NO2], total nitrogen [TN], phosphate [PO4], total phosphorus [TP], and suspended sediment concentration) and quantity (discharge) were measured using sensors and automated water samplers installed in surface drainage pipes at the EOF and laboratory analysis. We monitored both precipitation and irrigation events for three years, with the first two years in cover crop and the third year in baseline study. We used the data to evaluate (1) the effectiveness of cover crops in reducing nutrients and sediment losses, (2) differences in growing season and nongrowing season losses, and (3) changes in runoff water quality from irrigation tailwater compared to rainfall event runoff during the growing seasons. We also compared variations in pollutant loadings in the paired fields under identical crop and management practices for baseline monitoring. Cover crops effectively reduced concentrations of NO3-N and PO4-P from one field pair at the Caraway location. Generally, concentrations of pollutants were lower in the growing period (May through October) than in the nongrowing period (November through April), suggesting the need for practices such as cover crops to reduce winter loads. Runoff water quality after irrigation was not different from that after rainfall. These findings support the need for baseline studies before actual evaluation of conservation practices, and practices including cover crops at the field in winter.

Greenhouse Gas Emissions and Management Practices that Affect Emissions in US Rice Systems

Previous reviews have quantified factors affecting greenhouse gas (GHG) emissions from Asian rice ( L.) systems, but not from rice systems typical for the United States, which often vary considerably particularly in practices (i.e., water and carbon management) that affect emissions. Using meta-analytic and regression approaches, existing data from the United States were examined to quantify GHG emissions and major practices affecting emissions. Due to different production practices, major rice production regions were defined as the mid-South (Arkansas, Texas, Louisiana, Mississippi, and Missouri) and California, with emissions being evaluated separately. Average growing season CH emissions for the mid-South and California were 194 (95% confidence interval [CI] = 129-260) and 218 kg CH ha season (95% CI = 153-284), respectively. Growing season NO emissions were similar between regions (0.14 kg NO ha season). Ratoon cropping (allowing an additional harvestable crop to grow from stubble after the initial harvest), common along the Gulf Coast of the mid-South, had average CH emissions of 540 kg CH ha season (95% CI = 465-614). Water and residue management practices such as alternate wetting and drying, and stand establishment method (water vs. dry seeding), and the amount of residue from the previous crop had the largest effect on growing season CH emissions. However, soil texture, sulfate additions, and cultivar selection also affected growing season CH emissions. This analysis can be used for the development of tools to estimate and mitigate GHG emissions from US rice systems and other similarly mechanized systems in temperate regions.

Characterizing Irrigation Water Requirements for Rice Production from the Arkansas Rice Research Verification Program

This study investigated rice irrigation water use in the University of Arkansas Rice Research Verification Program between the years of 2003 and 2011. Irrigation water use averaged 747 mm (29.4 inches) over the eight years. A significant 40% water savings was reported for rice grown under a zero grade irrigation system (482 mm or 19.0 inches) compared to contour and straight levee systems. No differences in water use were found comparing contour levees (812 mm or 31.9 inches) and straight levees (815 mm or 32.0 inches). Arkansas producers implementing the multiple inlet water delivery practice on contour levee or straight levee systems, irrespective of soil type, did not realize a water savings. These results are in contrast to those in earlier studies and suggest that an educational effort may be helpful for those utilizing multiple inlet in Arkansas.