William E. Emmerich

Evaluation and Transferability of the Noah Land Surface Model in Semiarid Environments

Abstract This paper investigates the performance of the National Centers for Environmental Prediction (NCEP) Noah land surface model at two semiarid sites in southern Arizona. The goal is to evaluate the transferability of calibrated parameters (i.e., direct application of a parameter set to a “similar” site) between the sites and to analyze model performance under the various climatic conditions that can occur in this region. A multicriteria, systematic evaluation scheme is developed to meet these goals. Results indicate that the Noah model is able to simulate sensible heat, ground heat, and ground temperature observations with a high degree of accuracy, using the optimized parameter sets. However, there is a large influx of moist air into Arizona during the monsoon period, and significant latent heat flux errors are observed in model simulations during these periods. The use of proxy site parameters (transferred parameter set), as well as traditional default parameters, results in diminished model perfo...

Carbon fluxes on North American rangelands

Abstract Rangelands account for almost half of the earths land surface and may play an important role in the global carbon (C) cycle. We studied net ecosystem exchange (NEE) of C on eight North American rangeland sites over a 6-yr period. Management practices and disturbance regimes can influence NEE; for consistency, we compared ungrazed and undisturbed rangelands including four Great Plains sites from Texas to North Dakota, two Southwestern hot desert sites in New Mexico and Arizona, and two Northwestern sagebrush steppe sites in Idaho and Oregon. We used the Bowen ratio-energy balance system for continuous measurements of energy, water vapor, and carbon dioxide (CO2) fluxes at each study site during the measurement period (1996 to 2001 for most sites). Data were processed and screened using standardized procedures, which facilitated across-location comparisons. Although almost any site could be either a sink or source for C depending on yearly weather patterns, five of the eight native rangelands typically were sinks for atmospheric CO2 during the study period. Both sagebrush steppe sites were sinks and three of four Great Plains grasslands were sinks, but the two Southwest hot desert sites were sources of C on an annual basis. Most rangelands were characterized by short periods of high C uptake (2 mo to 3 mo) and long periods of C balance or small respiratory losses of C. Weather patterns during the measurement period strongly influenced conclusions about NEE on any given rangeland site. Droughts tended to limit periods of high C uptake and thus cause even the most productive sites to become sources of C on an annual basis. Our results show that native rangelands are a potentially important terrestrial sink for atmospheric CO2, and maintaining the period of active C uptake will be critical if we are to manage rangelands for C sequestration.

Coupling a grassland ecosystem model with Landsat imagery for a 10-year simulation of carbon and water budgets

In this study, high-spatial, low-temporal scale visible remote sensing data were used to calibrate an ecosystem model (EM) for semiarid perennial grasslands. The model was driven by daily meteorological data and simulated plant growth and water budget on the same time step. The model was coupled with a canopy reflectance model to yield the time course of shortwave radiometric profiles. Landsat Thematic Mapper (TM) and Enhanced TM Plus (ETM+) images from 10 consecutive years were used to refine the model on a spatially distributed basis. A calibration procedure, which minimized the difference between the normalized difference vegetation index (NDVI) simulated from the coupled model and measured by the TM and ETM+ sensors, yielded the spatial distribution of an unknown parameter and initial condition. Accuracy of model products, such as daily aboveground biomass, leaf area index (LAI) and soil water content, was assessed by comparing them with field measurements. The promising results suggest that this approach could provide spatially distributed information about both vegetation and soil conditions for day-to-day grassland management.

Hydrologic characteristics immediately after seasonal burning on introduced and native grasslands.

Fire on rangelands used as a management tool or as an unwanted wildfire removes vegetation cover. Vegetation cover is thought to be a dominate factor controlling surface runoff and erosion. Vegetation removal by a burn should have an immediate effect on runoff and erosion. Surface runoff and sediment production were evaluated immediately after fall and spring season burns at 2 locations with different soil and vegetation types for 2 years in southeastern Arizona. The evaluations were conducted with a rainfall simulator at 2 precipitation intensities. Immediately after a burn there was not a significant change in runoff and erosion, therefore, vegetation cover by itself was concluded not to be a dominate factor controlling surface runoff and erosion. The increase found in surface runoff and sediment production from the burn plots was not significantly greater than the natural variability for the locations or seasons. Significantly higher surface runoff and sediment production was measured in the fall season compared to the spring at 1 location.

Precipitation thresholds for CO2 uptake in grass and shrub plant communities on Walnut Gulch Experimental Watershed

In semiarid ecosystems, precipitation is the major driving force for carbon uptake and subsequent plant growth. The hypothesis for this study was that the timing, frequency, and precipitation amount would produce different precipitation thresholds for uptake of carbon dioxide in grass and shrub plant communities. Eight years of precipitation data were used to determine the amount needed for carbon dioxide uptake thresholds in spring and summer seasons. Bowen ratio energy balance systems were used to measure carbon dioxide and moisture fluxes. In spring at the shrub site, close to or above long-term average spring precipitation of 59 mm was required to produce an uptake response. At the grass site a minimum of 23 mm was needed to produce an uptake response, which was much less than the long-term precipitation average of 68 mm. At both sites, spring or multiple summer responses reduced the threshold values for a summer response. Summer threshold ranges for the shrub site were 57-94 mm with a spring response and 123-140 mm without. Grass site summer thresholds were 51-95 mm with a spring response and 80-148 mm without. Summer precipitation threshold values were higher than spring values relating to the high summer evapotranspiration demand. The influence and variability of precipitation timing and frequency on carbon dioxide uptake threshold values resulted in no definitive conclusions as to differences between the grass and shrub plant communities, except that the grass site had slightly lower thresholds. Precipitation timing and frequency influence on total carbon uptake in some situations were more important than total precipitation. The lower grass site threshold values, along with a shift in climate toward more frequent and smaller precipitation events, may give grass ecosystems a competitive growth advantage.

Nutrient dynamics of rangeland burns in southeastern Arizona.

Burning of vegetation generally increases surface runoff and erosion and potentially can change the nutrient dynamics of an ecosystem with loss of nutrients. Nitrogen, phosphorus, and potassium nutrient status of soil and aboveground biomass were determined before fall and spring burns and 1 year later at 2 different soil and vegetation type locations in southeastern Arizona. The evaluations were repeated in subsequent years to evaluate a year effect. Potential nutrient loss in surface runoff and sediment was assessed with rainfall simulations conducted immediately after prescribed burns and after a second burn one year later. Nutrient loss in the runoff water and sediment from burned areas was compared to paired unburned. The soil contained >98% of the total nutrient and was not significantly influenced by the burn treatment. The nutrient concentrations in the regrowth biomass were generally greater. Immediately after the first burn, nutrient loss in surface runoff and sediment was not affected by the burn treatment, but one location was greater than the other. After 1 year and a second burn, nutrient losses on the burn treatment were significantly greater than the unburned treatment and similar between locations. The nutrient loss in surface runoff was primarily associated with the sediment and influenced by an interaction between biomass and soil. The nutrient loss in runoff and sediment was small compared to the nutrient in the aboveground biomass and insignificant compared to the soil nutrient. The implication is that increased surface nutrient loss from burning could take place for many years before a significant amount of nutrient would be lost from the large soil pool and change the nutrient status of the ecosystem. Year and season were also important factors influencing nutrients in the soil, biomass, and in runoff and sediment losses, irrespective of a burn treatment effect.

Precipitation Regulates the Response of Net Ecosystem CO2 Exchange to Environmental Variation on United States Rangelands

Abstract Rangelands occupy 50% of Earths land surface and thus are important in the terrestrial carbon (C) cycle. For rangelands and other terrestrial ecosystems, the balance between photosynthetic uptake of carbon dioxide (CO2) and CO2 loss to respiration varies among years in response to interannual variation in the environment. Variability in CO2 exchange results from interannual differences in 1) environmental variables at a given point in the annual cycle (direct effects of the environment) and in 2) the response of fluxes to a given change in the environment because of interannual changes in biological factors that regulate photosynthesis and respiration (functional change). Functional change is calculated as the contribution of among-year differences in slopes of flux-environment relationships to the total variance in fluxes explained by the environment. Functional change complicates environmental-based predictions of CO2 exchange, yet its causes and contribution to flux variability remain poorly defined. We determine contributions of functional change and direct effects of the environment to interannual variation in net ecosystem exchange of CO2 (NEE) of eight rangeland ecosystems in the western United States (58 site-years of data). We predicted that 1) functional change is correlated with interannual change in precipitation on each rangeland and 2) the contribution of functional change to variance in NEE increases among rangelands as mean precipitation increases. Functional change explained 10–40% of the variance in NEE and accounted for more than twice the variance in fluxes of direct effects of environmental variability for six of the eight ecosystems. Functional change was associated with interannual variation in precipitation on most rangelands but, contrary to prediction, contributed proportionally more to variance in NEE on arid than more mesic ecosystems. Results indicate that we must account for the influence of precipitation on flux-environment relationships if we are to distinguish environmental from management effects on rangeland C balance.

Variability in germination rate among seed lots of Lehmann lovegrass.

The regeneration success of Lehmann lovegrass (Eragrostis lehmanniana Nees) in southern Arizona may be partially due to rapid germination during sporadic periods of available soil moisture. There is limited information regarding germination rate of Lehmann lovegrass but it is known that total germination response for this species is highly variable. Some of this variability may result from differences in the degree of mechanical scarification during harvest, threshing, and storage. Scarified and nonscarified seed from 7 seed lots were germinated over the water potential range of 0 to -1.16 MPa. Results showed that mechanical scarification increased total germination and germination rate. Mechanical scarification reduced variability among seed lots for germination rate, but increased variability for total germination. The rapid germination hypothesis may be valid for Lehmann lovegrass as long as seed numbers are not limiting. Of the scarified seed that germinated above a water potential of -0.4 MPa, at least 10% did so between days 1 and 2 of the study.

A remote sensing approach for estimating distributed daily net carbon dioxide flux in semiarid grasslands

[1] Semiarid systems compose a significant portion of the world’s terrestrial area and may play an important role in the global carbon cycle. A model was developed using the relation between surface reflectance and temperature obtained from satellite imagery to determine a Water Deficit Index (WDI) that estimated distributed plant transpiration, and by extension carbon dioxide (CO2) flux, for a point in time. Relationships were developed to scale these instantaneous flux measurements up to daytime estimates, which were then used to obtain measures of nighttime flux. Satellite images were acquired for a 5-year period (1996–2000) during which transpiration and net CO2 flux were measured for a semiarid grassland site in southeastern Arizona. Manual and automatic chamber data were also collected at the same site during the monsoon growing seasons of 2005 and 2006 and used to develop the relationship between and daytime and nighttime CO2 flux. Strong linear relationships were found between WDI-derived instantaneous and daytime net CO2 flux estimates (R 2 = 0.97), and between daytime and nighttime fluxes (R 2 = 0.88). These relations were used to generate maps of distributed total daily net CO2 flux. The error for the model was within the range of error inherent in the data sets used to create it and remained reasonable when used with WDI values less than 0.9. This study demonstrated that remote sensing can offer a physically based means of obtaining daily net CO2 flux in semiarid grasslands.

Partial and full dehydration impact on germination of 4 warm-season grasses.

Precipitation patterns in the arid southwest U.S. can be highly variable during the summer monsoon season. The ability of germinating seeds to withstand temporary periods of dehydration may determine their potential for successful regeneration under present and future climatic regimes. Germination with short-term hydration and dehydration sequences was compared to constant water potential germination for sideoats grama [Bouteloua curtipendula (Michaux) Torrey], buffelgrass [Cenchrus ciliaris L.], Lehmann lovegrass [Eragrostis lehmanniana Nees], and kleingrass [Panicum coloratum L.]. Seeds were imbibed at -0. MPa for 1 to 4 days, then either air dried or partially dehydrated d -3.0 MPa for 1 to 4 days before being returned to the initial imbibition solution for a total 14-day incubation-dehydration period. One day of imbibition at -0.2 MPa advanced germination to a stage that resulted in significant reductions (P 48% of the viable seeds to germinate after dehydration. Longer imbibition times also exhibited significant reductions in germination for buffelgrass and kleingrass. For kleingrass air-dried dehydration compared to -3.0 MPa produced significant reductions (P 1-day imbibition followed by dehydration seemed the critical time upon which a dramatic reduction in germination occurs.