Hydrological Behavior of Grasslands of the Sandhills of Nebraska: Water and Energy Balance Assessment from Measurements, Treatments and Modeling

Understanding energy and water balance processes in the Sandhills is crucial to assess the land-atmosphere feedback effects. The Sandhills located in western Nebraska covers a vast grassland ecosystem with limited variability in vegetation and soil. However, the combined effect of topography, land cover and micrometeorology by subjecting the land surface to various disturbances and treatments is rarely studied. The NOAH Land Surface Model was used to estimate net radiation, latent, sensible and ground heat fluxes as well as water balance components for two growing seasons between 2005 and 2006 in various plots at the Grasslands Destabilization Experimental site where these plots were subjected to four different treatments and located at two topographical locations namely high and low positions. The simulated results of net radiation and ground heat fluxes correlated well with measurements. While the amount of precipitation received was between 900 and 1000 mm for both seasons, on a daily and sub-daily time scale, the partitioning of net radiation into latent, sensible and ground heat fluxes showed high variability across the plots, primarily driven by vegetation and soil moisture. Total evapotranspiration and soil moisture averages suggested the influence of vegetation and timing of precipitation also in controlling various land surface processes in the Sandhills. This study provides a framework for using the LSM to quantify the feedback effects and emphasizes the importance of microtopography and land treatments in the model environment

Nutrient Cycling in Forage Production Systems

In most forage production systems, the nutrients needed for plant growth are provided by microbially mediated breakdown and release of plant-available mineral nutrients from dead plant tissues, livestock excreta, soil organic matter, and geochemically bound mineral forms. Even in fertilized forage systems, determining appropriate fertilizer application rates requires a systems approach on the part of the manager (e.g., Di and Cameron, 2000; Rotz et al., 2002). Fertilizer additions are simply one input in the system of inputs, outputs, pools, and fluxes that characterize nutrient cycling in a particular ecosystem

Evaluating the Simulation of a Simple Hydrology Model Using Long-Term Soil Moisture Measurements in the Nebraska Sand Hills

In this paper, we investigate soil moisture, evapotranspiration and other major water balance components over six sites in the Sand Hills of Nebraska during a 6-year period (1998-2003) using a hydrological model. We simulate water budget components including root zone soil moisture and found that model predictions of soil moisture compare reasonably well with observations for these sites. In the precipitation-limited Sand Hills, a moderate change in precipitation pattern from year to year is found to have profound effects on the fast response components of the hydrological cycle. Despite the homogeneity in terms of soil (sandy) and vegetation (grass), both the spatial and temporal variability in the estimated soil moisture, evapotranspiration, runoff and drainage suggests an active interaction among various hydrological processes in response to precipitation over this semi-arid region

Evaluating the Simulation of a Simple Hydrology Model Using Long-Term Soil Moisture Measurements in the Nebraska Sand Hills

In this paper, we investigate soil moisture, evapotranspiration and other major water balance components over six sites in the Sand Hills of Nebraska during a 6-year period (1998-2003) using a hydrological model. We simulate water budget components including root zone soil moisture and found that model predictions of soil moisture compare reasonably well with observations for these sites. In the precipitation-limited Sand Hills, a moderate change in precipitation pattern from year to year is found to have profound effects on the fast response components of the hydrological cycle. Despite the homogeneity in terms of soil (sandy) and vegetation (grass), both the spatial and temporal variability in the estimated soil moisture, evapotranspiration, runoff and drainage suggests an active interaction among various hydrological processes in response to precipitation over this semi-arid region

Smoothed Analysis of Tensor Decompositions

Low rank tensor decompositions are a powerful tool for learning generative models, and uniqueness results give them a significant advantage over matrix decomposition methods. However, tensors pose significant algorithmic challenges and tensors analogs of much of the matrix algebra toolkit are unlikely to exist because of hardness results. Efficient decomposition in the overcomplete case (where rank exceeds dimension) is particularly challenging. We introduce a smoothed analysis model for studying these questions and develop an efficient algorithm for tensor decomposition in the highly overcomplete case (rank polynomial in the dimension). In this setting, we show that our algorithm is robust to inverse polynomial error -- a crucial property for applications in learning since we are only allowed a polynomial number of samples. While algorithms are known for exact tensor decomposition in some overcomplete settings, our main contribution is in analyzing their stability in the framework of smoothed analysis. Our main technical contribution is to show that tensor products of perturbed vectors are linearly independent in a robust sense (i.e. the associated matrix has singular values that are at least an inverse polynomial). This key result paves the way for applying tensor methods to learning problems in the smoothed setting. In particular, we use it to obtain results for learning multi-view models and mixtures of axis-aligned Gaussians where there are many more "components" than dimensions. The assumption here is that the model is not adversarially chosen, formalized by a perturbation of model parameters. We believe this an appealing way to analyze realistic instances of learning problems, since this framework allows us to overcome many of the usual limitations of using tensor methods.Comment: 32 pages (including appendix

Impact of grassland conversion to forest on groundwater recharge in the Nebraska Sand Hills

Study region: Nebraska National Forest in the High Plains Aquifer, Nebraska Sand Hills, U.S.A. Study focus: This research aimed to investigate the effects of grassland conversions to forest on recharge rates in a century-old experimental forest. The DiffeRential Evolution Adaptive Metropolis (DREAMZS) global optimization algorithm was used to calibrate the effective soil hydraulic parameters from observed soil moisture contents for 220 cm deep uniform soil profiles. The historical recharge rates were then estimated by applying the numerical model HYDRUS 1-D for simulation of two plots representing grasslands and dense pine forest conditions. New hydrological insights: The results indicate that conversion from grasslands to dense pine forests led to vegetation induced changes in soil hydraulic properties, increased rooting depth, and greater leaf area index, which together altered the water budget considerably. The impacts of land use change, expressed in percent of gross precipitation, include a 7% increase in interception associated with an increase in leaf area index, a nearly 10% increase in actual evapotranspiration, and an overall reduction of groundwater recharge by nearly 17%. Simulated average annual recharge rates decreased from 9.65 cm yr−1 in the grassland to 0.07 cm yr−1 in the pine plot. These outcomes highlight the significance of the grassland ecology for water resources, particularly groundwater recharge, in the Nebraska Sand Hills and the overall sustainability and vitality of the High Plains Aquifer

Biodiversity and decomposition in experimental grassland ecosystems

We examined the impact of biodiversity on litter decomposition in an experiment that manipulated plant species richness. Using biomass originating from the experimental species richness gradient and from a species used as a common substrate, we measured rates of decomposition in litterbags in two locations: in situ in the experiment plots and in an adjacent common garden. This allowed us to separate the effects of litter quality and decomposition location on decomposition. We found that plant species richness had a significant, but minor negative effect on the quality (nitrogen concentration) of the biomass. Neither litter type nor location had a consistent effect on the rate of carbon and nitrogen loss over a 1-year period. Thus, the increased productivity and corresponding lower soil available nitrogen levels observed in high diversity plots do not lead to faster litter decomposition or faster nitrogen turnover. This supports the hypothesis that increased productivity corresponding with higher species richness results in increased litter production, higher standing litter pools and a negative feedback on productivity, because of an increased standing nitrogen pool in the litter

CLUES TO THE MEDIEVAL DESTABILIZATION OF THE NEBRASKA SAND HILLS, USA, FROM ANCIENT POCKET GOPHER BURROWS

The Nebraska Sand Hills are a stabilized dune field in the central United States that reflect past conditions of drought. The most recent drought, known as the Medieval Climatic Anomaly, occurred from A.D. 900 to A.D. 1300 and had an enormous effect on the thriving prairie ecosystem, which included large populations of the plains pocket gopher (Geomys bursarius). Burrows of these organisms across a paleosol-eolian sand boundary in the Sand Hills indicate abrupt climate change during the transition from stabilized to active dune field and from humid to arid conditions. Medieval gophers tunneled at greater depths below the surface than do modern gophers, indicating the behavioral changes these animals underwent to survive during the transition. The gophers were likely surviving on roots remaining in the underlying soil as it was buried by sand; they tunneled .1 m up to the surface to deposit mounds of excavated soil and sand. Most of the burrows occur in areas of low-angle bedding, suggesting loss of vegetation occurred first on the crests of the newly formed dunes while vegetation persisted in the interdunes. Optically stimulated luminescence dates from a dune containing ancient gopher burrows are nearly identical throughout the height of the dune, indicating rapid accumulation of sand. As accumulation of sand was rapid, vegetative loss must also have occurred quickly, though not in a uniform pattern across the region. Pocket gophers were apparently able to survive in areas of remaining vegetation for a short time, but in a relatively short period of time, they were unable to reach their food sources and were forced ultimately to abandon the uplands in the region

The Role of Litter Quality Feedbacks in Terrestrial Nitrogen and Phosphorus Cycling

Many studies in ecosystem ecology argue for strong control of litter quality over nitrogen (N) cycling. We developed a model for temperate grasslands to test the importance of litter quality in decomposition for N and phosphorus (P) cycling based on the following premises. First, terrestrial N and P cycling differ fundamentally because N is a structural component of the soil organic matter (SOM), whereas P is not. Secondly, SOM has a much lower C:N ratio than litter inputs. Thirdly, litter decomposition follows an exponential decay with 20% of the original litter mass turning into SOM. Fourth, litter N concentration shows an exponential increase during decomposition, whereas P does not change and is released proportionally to the litter mass. Based on these premises we constructed a model which shows that 0.75% N is a critical initial litter concentration at which concentration all N is immobilized and no N is released from the litter. Thus at 0.75% N of the litter all net N mineralization is through SOM decomposition and not through litter decomposition. Phosphorus, in contrast, is primarily released in the early stages of litter decomposition. Empirical tests of these model predictions support the applicability of the model to temperate grassland ecosystems. This model predicts that N mineralization from SOM is much more important than mineralization from litter and that plant litter quality differences alone cannot explain ecosystem N cycling patterns. Phosphorus, in contrast, does cycle largely through litter decomposition, and plant litter quality differences are the dominant factor in determining ecosystem P cycling feedbacks