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Second generation biofuels, such as perennial grasses, have potential to provide biofuel feedstock while growing on degraded land with minimal inputs. Perennial grasses have been reported to sequester large amounts of soil organic carbon (SOC) in the Midwestern United States (USA). However, there has been little work on biofuel and carbon sequestration potential of perennial grasses in the Southeastern US. Biofuel productivity for dryland Miscanthus × gigantus and irrigated maize in Georgia, USA were quantified using eddy covariance observations of evapotranspiration (ET) and net ecosystem exchange (NEE) of carbon. Miscanthus biomass yield was 15.54 Mg ha−1 in 2015 and 11.80 Mg ha−1 in 2016, while maize produced 30.20 Mg ha−1 of biomass in 2016. Carbon budgets indicated that both miscanthus and maize fields lost carbon over the experiment. The miscanthus field lost 5 Mg C ha−1 in both 2015 and 2016 while the maize field lost 1.37 Mg C ha−1 for the single year of study. Eddy covariance measurement indicated that for 2016 the miscanthus crop evapotranspired 598 mm and harvest water use efficiencies ranged from 6.95 to 13.84 kg C ha−1 mm−1. Maize evapotranspired 659 mm with a harvest water use efficiency of 19.12 kg C ha−1 mm−1. While biomass yields and gross primary production were relatively high, high ecosystem respiration rates resulted in a loss of ecosystem carbon. Relatively low biomass production, low water use efficiency and high respiration for Miscanthus × gigantus in this experiment suggest that this strain of miscanthus may not be well-suited for dryland production under the environmental conditions found in South Georgia USA.

Field experiments offer the most acceptable approach to quantifying agricultural fumigant emissions but there is an absence of replicated field data in reported literature. Air concentration profiles of 1,3-dichloropropene (1,3-D) were determined on duplicate masts above the center of a treated field over 14 days. Meteorological parameters were also measured. Three meteorological approaches were then used to determine the total and flux density emissions of 1,3-D. Across the three calculation methods, the averages of the duplicated measurements showed total emission losses of cis 1,3-D ranging from 27% to 36% and of trans 1,3-D ranging from 18% to 24%. The replicate measurements differed by between 1.6 and 7.7 percentage points, which we consider to be excellent replicability. Flux densities over time showed maximum emissions during the first nighttime and early morning of the day following application. A general declining trend in emission fluxes was accompanied by nighttime peaks. Flux density curves during the experiment showed excellent agreement between replicates, with linear regression of the two data sets yielding r2 values of 0.95–0.98 and slopes of 1.01–1.17. To our knowledge, this is the first time that replicated fumigant fluxes have been reported. The high degree of replicability indicates the robustness of the approaches and lends credence to previous non-replicated flux data.

The eddy covariance method is regularly used for measuring gas fluxes over agricultural fields and natural ecosystems. For many applications, it is desirable to partition the measured fluxes into constitutive components: the water vapor flux into transpiration and direct evaporation components, and the carbon dioxide flux into photosynthesis and respiration components. The flux variance similarity (FVS) partitioning method is based on flux variance similarity relationships and correlation analyses of high-frequency eddy covariance data ( Scanlon and Sahu, 2008 , Scanlon and Kustas, 2010 , Scanlon and Kustas, 2012 ). The FVS method is relatively complex computationally, and that complexity has likely been an impediment to greater use and testing of the procedure. In this work, we present a new algebraic solution to the key computational task in the partitioning algorithm, which significantly simplifies the FVS method. We also introduce Fluxpart, a free and open source Python 3 module that implements the FVS partitioning procedure. Example flux partitioning calculations are presented.

Current approaches to scheduling crop irrigation using reference evapotranspiration (ET 0 ) recommend using a dual-coefficient approach using basal (K cb ) and soil (K e ) coefficients along with a stress coefficient (K s ) to model crop evapotranspiration (ET c ), [e.g. ET c = (K s *K cb + K e )*ET 0 ]. However, determining K s , K cb , and K e from the combined evapotranspiration (ET) is challenging, particularly for K e , and a new method is needed to more rapidly determine crop coefficients for novel cultivars and cultivation practices. In this study, we partition eddy covariance ET observations into evaporation (E) and transpiration (T) components using correlation structure analysis of high frequency (10–20 Hz) observations of carbon dioxide and water vapor (Scanlon and Sahu, 2008) at three irrigated agricultural sites. These include a C4 photosynthetic-pathway species (sugarcane— Sacharum officinarum L.) and a C3 pathway species (peach —Prunus persica ) under sub-surface drip and furrow irrigation, respectively. Both sites showed high overall K c consistent with their height (>4 m). The results showed differences in K e , with the sub-surface drip-irrigated sugarcane having a low K e (0.1). There was no significant relationship (r 2 cb *K s , indicating that there was no stress (K s = 1), while the peach orchard showed mid-season declines in K cb *K s when VWC declined below 0.2. Partitioning of K c into K cb and K e resulted in a better regression (r 2 = 0.43) between the Normalized Differential Vegetation Index (NDVI) and K cb in sugarcane than between NDVI and K c (r 2 = 0.11). The results indicate the potential for correlation structure flux partitioning to improve crop ET coefficient determination by improved use of eddy covariance observations compared to traditional approaches of lysimeters and microlysimeters and sap flow observations to determine K c , K e , K s , and K cb .

A three-year study was conducted to assess the ability of satellite-based vegetation index (VI) images to track evapotranspiration over wheat. While the ability of using VIs, notably with the Normalized Difference Vegetation Index (NDVI), to track vegetation growth has been well established, the operational capability to accurately estimate the crop coefficient (Kc) and crop evapotranspiration (ETc) at farm-scale from spaceborne platforms has not been widely studied. The study evaluated wheat ET over 7 sites between 2016 and 2019 in Yuma and Maricopa, Arizona, USA estimated by using Sentinel 2 and Venus satellites to map NDVI time-series for entire wheat cropping seasons, December to June. The basal crop coefficient (Kcb) was modeled by the NDVI time-series and the daily FAO56 reference ETo was obtained by near-by weather network stations. Eddy covariance (EC) stations in each field observed ETc during the same seasonal periods, and applied irrigation amounts were logged. The experiment found that remote sensing of NDVI and modeled Kcb accurately estimated Kc and crop ET during mid-season through senescence in most cases. However, NDVI-based estimation performed less well during early season (

Water availability is a major limiting factor for sustainable production of potential biofuel crops in Maui, Hawaii, USA. It is essential to improve regional, near-real time estimates of crop water use to facilitate optimal water management. Satellite remote-sensing offers multiple methods to estimate water management, however, most approaches assume a spatially homogeneous regional meteorology, which does not apply in Hawaii due to strong orographically-induced variations in climate. Canopy ground cover ( f c ) and spectral reflectance were measured over two sugarcane fields on the satellite Landsat 7 overpass dates. A strong linear relationship between satellite-based Normalized Difference Vegetation Index (NDVI) values and f c was found ( R 2 = 0.97). Canopy ground cover was also found to be highly correlated to crop coefficient ( K c ) which was calculated from reference evapotranspiration (ET 0 ) and sugarcane crop evapotranspiration (ET c ) observations from the Eddy Covariance towers. Spatial ET 0 for the entire sugarcane plantation was determined from an automated weather station network previously installed by the commercial grower of the plantation. Regressions were used to convert the satellite NDVI to K c maps, and then the satellite-based ET c maps were created from the satellite-based K c maps and ET 0 values computed and interpolated from the weather station network. To compare with the satellite-based crop coefficient/reference ET method, ET c and K c were also estimated using a satellite-based surface energy balance technique, the METRIC model. These two satellite-based methods were correlated fairly well ( R 2 = 0.84 for K c and 0.81 for ET c ). Compared to the METRIC model, the satellite-based crop coefficient/reference ET method may be more practicable for estimating crop water use in Maui, where ground-based ET 0 measurements are available through the on-site automated weather station network with right choice of reference ET equation.

Sugarcane has been a major agronomic crop in Hawaii with an unique, high-yield, two-year production system. However, parameters relevant to advanced, cellulosic biofuel production, such as net ecosystem productivity (NEP) and radiation use efficiency (RUE), have not been evaluated in Hawaii under commercial production. Recent demand potential has rekindled interest in Hawaiian grown biofuels; as such, there is a need to understand productivity under changing climate and agronomic practices. To this end, we established two eddy covariance towers in commercial sugarcane fields in Maui, Hawaii to evaluate the carbon balance and RUE of sugarcane under contrasting elevations and soil types. We combined the tower observations with biometric and satellite data to assess RUE in terms of net biomass accumulation and daily gross primary production. High, sustained net NEP was found in both fields (cumulative NEP 4.23–5.37 × 10 3 g C m −2 over the course of the measurement period). Biomass RUE was statistically similar for both fields (1.15–1.24 g above ground biomass per MJ intercepted solar irradiance). Carbon accumulated in both fields at nearly the same rate with differences in cumulative biomass due to differing crop cycle lengths; cumulative gross primary productivity and ecosystem respiration were higher in the lower elevation field. Contrary to previous studies in Hawaiian sugarcane, we did not see a large decrease in NEP or increase in ecosystem respiration in the 2nd year, which we attributed to suppressed decomposition of dead cane stalks and leaves due to drip irrigation and drought. Biomass RUE also showed little decline in the 2nd year. The results show that Hawaiian sugarcane has a higher productivity than sugarcane grown in other regions of the world and also suggests that a longer (>12 months) growing cycle may be optimal for biomass production.

The lack of energy closure has been a longstanding issue with Eddy Covariance (EC). Multiple mechanisms have been proposed to explain the discrepancies in energy balance including diurnal energy storage changes, advection of energy, and larger scale turbulent processes that cannot be resolved by field EC. To investigate the energy balance issue, we used a year of data from paired EC towers in irrigated sugarcane in Maui, Hawai’i, USA. The towers were in identical crops and cultivation practices and had similar climate with the notable exception of wind. One tower was in a location where nearby orographic features funneled Trade Winds, resulting in sustained, continuous turbulence. The other was in a leeward location with less turbulence, particularly at night (u*). We found significantly improved closure (8.5–10%) at both sites using daily sums of Available Energy in closure regressions as opposed to 30 min data, illustrating the importance of storage terms. The energy budget closed for both fields when only days with continuous turbulence (all 30 min u* > critical u*) were considered, with significantly larger uncertainty in the leeward field (±13%) due to the small number of days (n = 13) with this condition. Significant energy imbalance appeared in both fields with even 30 min of subcritical turbulence in a day, and each field had different turbulence-closure patterns. Closure with continuous turbulence was sensitive to choice of critical u*; an arbitrary u* of 0.1 m s−1 resulted in non-closure. The results show the value of paired EC towers in contrasting turbulence conditions to assess energy budget closure.

We evaluate a new approach to estimate regional evapotranspiration (ET) across a montane, Mediterranean climate gradient in the San Jacinto and Santa Rosa Mountains of Southern California. Spatially distributed evaporative fraction (EF) measurements were made monthly from October 2008 to September 2009 at 54 locations across an elevational gradient using a mobile measurement platform, called the Regional Evaporative Fraction Energy Balance (REFEB) method. We used these measurements and the Enhanced Vegetation Index (EVI) from MODerate resolution Imaging Spectroradiometer (MODIS) observations to derive EF at a regional scale. We converted EF to monthly ET using remote-sensing based observations of available energy. We compared the REFEB ET estimates, along with modified Priestly–Taylor (PT) ET estimates driven by MODIS data against four eddy covariance (EC) towers and eight gauged catchments. Both of the satellite-based ET estimates were highly correlated with tower ET observations (r 2 = 0.66 for REFEB and 0.95 for PT). The PT MODIS approach overestimated ET compared to precipitation estimates and stream gauge measurements, while REFEB ET was moderately lower than PT ET. The annual regional REFEB ET (193 mm) was 87 mm less than precipitation (280 mm). REFEB ET underestimated EC tower ET (regression slope = 0.78, p

Regional estimates of evapotranspiration (ET) are needed for environmental analysis and management purposes, yet can be difficult to obtain. Current methods for determining regional ET have spatial, temporal, methodological, and/or logistical limitations that affect their usefulness. To address these gaps, we developed a surface mobile measurement technique, the Regional Evaporative Fraction Energy Balance platform (REFEB), which measures evaporative fraction (EF) and water use efficiency (WUE) using a truck operating on a public right of way. REFEB can measure EF and WUE at 25 or more locations per day, which allows for rapid, dense, and spatially distributed sampling of fields across a region. We assessed the accuracy of the field measurements of EF and WUE with REFEB by comparing them to an Eddy covariance (EC) and Bowen ratio energy balance (BREB) tower. This site validation showed that REFEB has error and uncertainty similar to previous BREB approaches. We then used empirical relationships between field measurements and remote sensing vegetation indices to derive regional maps of EF. We combined these EF maps with satellite observations of net radiation to derive monthly and annual calculations of ET at a 250 m resolution during calendar year 2004 in the Imperial Valley, California, a major agricultural region that is dependent upon irrigation and which has a well constrained water budget. We then summed ET for the Imperial Valley and compared the result to a surface water budget based on irrigation and drainage measurements, which showed good annual and seasonal agreement. These results indicate REFEB produces accurate field measurements of EF and WUE, which can be scaled to estimate regional ET at time scales ranging from less than a week to annual sums.