High Net Radiation Research Articles

Improved 30‐m Evapotranspiration Estimates Over 145 Eddy Covariance Sites in the Contiguous United States: The Role of ECOSTRESS, Harmonized Landsat Sentinel‐2 Imagery, Climate Reanalysis, and Deep Neural Network Postprocessing

AbstractThis study developed and evaluated 30‐m daily evapotranspiration (ET) estimates using the Priestley‐Taylor Jet Propulsion Laboratory (PT‐JPL) model with ECOSTRESS, Moderate MODIS, harmonized Landsat Sentinel‐2 (HLS) imagery, ERA5‐Land reanalysis, and eddy covariance measurements. The new daily 30‐m ET showed significantly improved performance (overall, r = 0.8, RMSE = 1.736, KGE = 0.466) at 145 EC sites over contiguous United States compared to the current 70‐m ECOSTRESS ET (overall, r = 0.485, RMSE = 4.696, KGE = −0.841). A deep neural network postprocessing model trained with ET measurements from EC sites further improved the performance on test sites that were not used for model training (overall, r = 0.842, RMSE = 0.88, KGE = 0.792). The 30‐m ET estimation biases were significantly related to the biases in the upwelling longwave (RUL) and downwelling shortwave radiation (RDS) inputs, with ET estimates driven by MODIS radiation showing higher biases compared to those driven by ERA5‐Land radiation. The error diagnosis using random forest indicates that ET biases tend to be larger under higher ET estimates, and RUL and RDS were the primary contributors to the high bias at the higher ET ranges, with partial dependence plots revealing that the estimation biases tend to be higher under more humid environment, denser vegetation covers, and high net radiation conditions. In conclusion, higher spatial resolution satellite imagery of vegetation characteristics and higher temporal resolution radiation data, combined with continent‐wide EC measurements and deep learning, provided substantial added value for improving ET estimations at the field scale (30‐m).

Practical Considerations and Limitations of Using Leaf and Canopy Temperature Measurements as a Stomatal Conductance Proxy: Sensitivity across Environmental Conditions, Scale, and Sample Size.

Stomatal conductance (gs) is a crucial component of plant physiology, as it links plant productivity and water loss through transpiration. Estimating gs indirectly through leaf temperature (Tl) measurement is common for reducing the high labor cost associated with direct gs measurement. However, the relationship between observed Tl and gs can be notably affected by local environmental conditions, canopy structure, measurement scale, sample size, and gs itself. To better understand and quantify the variation in the relationship between Tl measurements to gs, this study analyzed the sensitivity of Tl to gs using a high-resolution three-dimensional model that resolves interactions between microclimate and canopy structure. The model was used to simulate the sensitivity of Tl to gs across different environmental conditions, aggregation scales (point measurement, infrared thermometer, and thermographic image), and sample sizes. Results showed that leaf-level sensitivity of Tl to gs was highest under conditions of high net radiation flux, high vapor pressure deficit, and low boundary layer conductance. The study findings also highlighted the trade-off between measurement scale and sample size to maximize sensitivity. Smaller scale measurements (e.g., thermocouple) provided maximal sensitivity because they allow for exclusion of shaded leaves and the ground, which have low sensitivity. However, large sample sizes (up to 50 to 75) may be needed to differentiate genotypes. Larger-scale measurements (e.g., thermal camera) reduced sample size requirements but include low-sensitivity elements in the measurement. This work provides a means of estimating leaf-level sensitivity and offers quantitative guidance for balancing scale and sample size issues.

Bioclimatic Zoning for Sheep Farming through Geostatistical Modeling in the State of Pernambuco, Brazil.

The Intergovernmental Panel on Climate Change (IPCC) has pointed out the high vulnerability of developing countries to climate change, which is expected to impact food and income security. Sheep farming is one of the main animal productions among the families located in the most vulnerable regions of the semiarid region of Pernambuco state, a Brazilian territory known for its high temperatures, low relative humidity, and high net solar radiation. Therefore, the objective of this study was to identify different regions of Pernambuco that may be more suitable for different breeds of sheep, based on non-parametric statistics and kriging maps of the temperature and humidity index (THI). THI values were determined based on mean annual temperature and wind speed extracted from the TerraClimate remote sensing database. Pernambuco state presented THI values ranging from 66 to 79, with the hair breeds having a high potential for exploitation in almost all territories, including the main meat-producing breeds. The East Friesian breed, a high milk producer, would be well suited to the Agreste mesoregion, a territory that, like the Pajeú and Moxotó microregions, also proved favorable for the introduction of three wool breeds (Suffolk, Poll Dorset, and Texel) known as major meat producers. The kriging maps of the THI values successfully allowed the identification of strategic development regions of Pernambuco state with high potential for sheep breeding.

Drivers of spatiotemporal patterns of surface water inputs in a catchment at the rain-snow transition zone of the water-limited western United States

Spatial and temporal dynamics of rainfall and snowmelt (i.e., surface water inputs, SWI) control soil moisture, groundwater recharge, and streamflow at annual, seasonal, and event scales. In the rain-snow transition zone, comprising a large portion of the mountainous western United States, there is limited understanding of the sensitivity of spatiotemporal SWI dynamics across hydrologically variable water years (WYs). We modeled rainfall and snowpack dynamics in a small headwater catchment (1.8 km2) spanning the rain-snow transition in southwestern Idaho, USA, for two hydrologically distinct WYs (2011 and 2014). In wet WY 2011 and dry WY 2014, total precipitation drove spatial variability in annual SWI. Snow drifts generated more SWI (901–2080 mm) than high-elevation scour zones (442–640 mm), which generated less SWI than mid-elevation, non-drift locations (452–784 mm). Seasonally, energy fluxes differed most during the snowmelt period, where higher net radiation at lower elevations and south-facing slopes drove SWI production. At the rain-on-snow (ROS) event scale, higher elevations and north-facing slopes generated 15–20 % of annual SWI, due mainly to higher turbulent fluxes. The most productive ROS events occurred after peak snow water equivalent (SWE), when rainfall fell onto ripe snowpacks. Snow drift locations were less susceptible to melt during ROS events, offset by the larger cold content and snowpack mass. Thus, catchment water resources depend on SWI magnitude, location, and timing, which are moderated by drift persistence at all temporal scales. As the climate warms, shifts in spatiotemporal SWI distribution are expected with declines in snowfall and snowfall redistribution in this area.

Significant water stress on gross primary productivity during flash droughts with hot conditions

High vapor pressure deficit (VPD) and low soil moisture (SM) are regarded as atmospheric and soil water stress on the ecosystems respectively, but the two factors are influencing ecosystems through different mechanisms. The relative influences of two sources of water stress during droughts are difficult to separate, especially for flash droughts that develop rapidly with strong land-atmospheric coupling. The dominant factors that expose ecosystems to water stress during droughts are controversial, which hinders the understanding of flash drought impacts and the related adaptation practices. Therefore, here we decouple SM and VPD stress on gross primary productivity (GPP) during flash droughts using hourly observations from 35 global FLUXNET stations with long-term eco-hydro-meteorological records. The results show that 52% of the stations have significant water stress on GPP during flash droughts, where low SM and high VPD are dominant for 46% and 6% of the stations respectively. For subsets of flash droughts with hot conditions that are defined by concurrent high temperature and above normal solar radiation (or defined by concurrent high net radiation larger than 400 W/m2), low SM and high VPD dominate water stress for 37% and 49% (31% and 50%) of the stations respectively. In addition, low SM and high VPD dominate water stress on the GPP normalized by solar radiation for 14% and 80% of the stations respectively, but the role of SM might be underestimated if flash droughts penetrate in deep soil. As compared with forest ecosystem productivity, non-forest ecosystem productivity declines quickly in response to soil water stress during the onset stage of flash droughts. Our results suggest an intensified water stress on ecosystem productivity and the dominance of high VPD stress during flash droughts with hot conditions, although the soil water stress is nontrivial for unconditional flash droughts.

Eucalyptus obliqua tall forest in cool, temperate Tasmania becomes a carbon source during a protracted warm spell in November 2017

Tasmania experienced a protracted warm spell in November 2017. Temperatures were lower than those usually characterising heatwaves. Nonetheless the warm spell represented an extreme anomaly based on the historical local climate. Eddy covariance measurements of fluxes in a Eucalyptus obliqua tall forest at Warra, southern Tasmania during the warm spell were compared with measurements in the same period of the previous year when temperatures were closer to average. Compared with previous year, the warm spell resulted in 31% lower gross primary productivity (GPP), 58% higher ecosystem respiration (ER) and the forest switching from a carbon sink to a source. Significantly higher net radiation received during the warm spell was dissipated by increased latent heat flux, while canopy conductance was comparable with the previous year. Stomatal regulation to limit water loss was therefore unlikely as the reason for the lower GPP during the warm spell. Temperatures during the warm spell were supra-optimal for GPP for 75% of the daylight hours. The decline in GPP at Warra during the warm spell was therefore most likely due to temperatures exceeding the optimum for GPP. All else being equal, these forests will be weaker carbon sinks if, as predicted, warming events become more common.

Are northern German Scots pine plantations climate smart? The impact of large-scale conifer planting on climate, soil and the water cycle

Increasing temperatures and rising atmospheric vapor pressure deficits are exposing forests around the globe to increasing drought and heat stress, demanding a shift to climate-smart forestry for increasing the stress resistance and resilience of production forests and to enhance their climate change mitigation potential. Based on measurements in paired pine and beech forests and the review of literature data, we analyse the biophysical consequences and the carbon cycle impact of large-scale Scots pine (Pinus sylvestris L.) plantations in northern Germany in the face of a warming and aridifying climate. We quantified canopy surface albedo and surface temperature, evapotranspiration and deep seepage, carbon (C) storage in biomass and soil and annual C sequestration, and soil acidification of pine plantations in comparison to beech forests (Fagus sylvatica L.), the natural forest vegetation. We find that near-infrared (NIR, 700–3000 mn) canopy surface albedo is higher by 5.2 percentage points during summer over beech as compared to pine forest, resulting in a 9 % higher net radiation and a 0.6 K higher surface temperature of the pine canopy. Deep seepage is on average by 68 mm yr−1 smaller under pine than beech forest (66 mm yr−1 vs. 134 mm yr−1) due to the higher evapotranspiration of pine. C storage in biomass and soil is by ∼ 48 Mg C ha−1 higher in beech than pine forests, reflecting the higher productivity of beech, demonstrating an unfavorably low C sequestration potential of Scots pine plantations. We conclude that the large-scale Scots pine plantations in northern Germany (>1.7 million ha) are neither environmental-friendly nor climate smart, given their enhancement of climate-warming, low climate change mitigation potential, and negative effect on groundwater recharge. Replacing pine plantations by beech (or other hardwood) forests in northern Germany and adjacent regions is urgently needed for achieving the goals of climate-smart forestry.

Estimating the spatial distribution of evapotranspiration within the Pra River Basin of Ghana

It is important in water resource planning to accurately estimate the spatial distribution of evapotranspiration (ET) as an input parameter for hydrological studies. Although, conventional pan evaporation, lysimetric and eddy covariance techniques have been used, they only estimate point values. Hence, this study aimed at estimating the spatial distribution of ET within the Pra River Basin (a forest ecological zone) of Ghana, using cloud-free Landsat 8 (OLI/TIRS) satellite images employing the SEBAL methodology. The study further estimates the spatial distribution ET in relation to major climatic variables, Land Use Land Cover (LULC) types and energy balance components. The overall spatial distribution of ET had a mean value of 5.63 mm/day. Spatial distribution of ET (mm/day) for water body (5.51–7.81) and uncultivated forest (5.10–7.71) were high, while moderately average values were observed for logged forest (4.80–7.51). Settlement and bare landscapes observed low rates ((2.05–5.10) mm/day). Spatially, ET was higher in the upper western, central and the eastern parts of the basin, but lower in the northern part and pockets of areas at the southern part of the basin where settlement/bare landscape and logged forest dominate. Areas with high temperature and high solar radiation experiences high ET, while low wind speed, low to average temperature and solar radiation areas experience low ET. Also, areas with both high net radiation and ground heat flux but low to average sensible heat flux experiences high ET and vice versa. Linear regression analysis showed good fit with slope of 0.76 and R2 of 0.93 indicating that 93 % of the variations in observed field measurement of ET fitted perfectly well with ET distributions generated by the SEBAL model.

Water-stress-induced breakdown of carbon–water relations: indicators from diurnal FLUXNET patterns

Abstract. Understanding of terrestrial carbon and water cycles is currently hampered by an uncertainty in how to capture the large variety of plant responses to drought. In FLUXNET, the global network of CO2 and H2O flux observations, many sites do not uniformly report the ancillary variables needed to study drought response physiology. To this end, we outline two data-driven indicators based on diurnal energy, water, and carbon flux patterns derived directly from the eddy covariance data and based on theorized physiological responses to hydraulic and non-stomatal limitations. Hydraulic limitations (i.e. intra-plant limitations on water movement) are proxied using the relative diurnal centroid (CET*), which measures the degree to which the flux of evapotranspiration (ET) is shifted toward the morning. Non-stomatal limitations (e.g. inhibitions of biochemical reactions, RuBisCO activity, and/or mesophyll conductance) are characterized by the Diurnal Water–Carbon Index (DWCI), which measures the degree of coupling between ET and gross primary productivity (GPP) within each day. As a proof of concept we show the response of the metrics at six European sites during the 2003 heat wave event, showing a varied response of morning shifts and decoupling. Globally, we found indications of hydraulic limitations in the form of significantly high frequencies of morning-shifted days in dry/Mediterranean climates and savanna/evergreen plant functional types (PFTs), whereas high frequencies of decoupling were dominated by dry climates and grassland/savanna PFTs indicating a prevalence of non-stomatal limitations in these ecosystems. Overall, both the diurnal centroid and DWCI were associated with high net radiation and low latent energy typical of drought. Using three water use efficiency (WUE) models, we found the mean differences between expected and observed WUE to be −0.09 to 0.44 µmol mmol−1 and −0.29 to −0.40 µmol mmol−1 for decoupled and morning-shifted days, respectively, compared to mean differences −1.41 to −1.42 µmol mmol−1 in dry conditions, suggesting that morning shifts/hydraulic responses are associated with an increase in WUE, whereas decoupling/non-stomatal limitations are not.

Vineyard microclimate and yield under different plastic covers.

The use of plastic cover in vineyards minimizes effects of adverse weather conditions. The northwest of São Paulo State is one of the largest grape producing regions in Brazil; however, few studies investigate the effects of different plastic covers on vineyards in this region. This study compared the effect of black shading screen (BSS) and braided polypropylene film (BPF) on BRS Morena vineyard microclimate, grown on an overhead trellis system in the northwestern São Paulo. The experiments were carried out during three growing seasons (2012-2014). BSS allowed superior incoming solar radiation (SR) transmissivity, resulting in higher net radiation (Rn), and higher ratio between photosynthetically active (PAR) and SR. No differences were observed between the average air temperatures (T) and relative humidity (RH) of covered environments (BPF and BSS) and outside condition (automatic weather station-AWS), due to high air circulation, despite wind speed (WS) reduction caused by plastic covers. BPF provided better conditions for vineyard growth with higher fruit yield than vineyard under BSS regarding the number of shoots with bunches per plant, bunch and stem weights, longitudinal diameter of berries, quantity of fertile buds per shoot, and yield per shoot and per plant. BPF covers also influenced leaf size and growth speed of plants in vineyards.

Study of energy partitioning and its feedback on the microclimate over different surfaces in an arid zone

Model simulations show that land use and land cover changes(LUCC) may alter surface energy budget and influence surface microclimate, but up to now, it still lacks of sufficient observations for explaining the mechanism of climate change brought about by LUCC. Grasslands and shrub lands are typical land covers in the mid-latitude arid zone of the northern hemisphere. The data used in this paper was collected from four sites which are related to grassland, open shrubland, savanna and closed shrubland, and all located in New Mexico, USA. The four sites are near each other and have the same background in climate and weather. Thus, the difference in surface energy partitioning over the four surfaces is induced by different land processes, which was explained in our study. The paper also analyzed the feedbacks of different land surface parameters and energy partitioning for the surface microclimate.We find that the leaf area index(LAI) and surface roughness of the four sites increases from the desert grassland to the closed shrubland. The difference in vegetation structures and functions also affects aerodynamic resistance and surface resistance to heat transfer and the resistances exhibit larger over sparse surfaces. Generally, the sites with high vegetation cover have higher net radiation, sensible and latent heat fluxes, particularly in the growing season. In addition, the contributions of impacting factors to the turbulent fluxes are diagnosed by Penman-Monteith equation and a mathematical formula combining net radiation with Bowen ratio. Compared to the desert grassland, the variations in net radiation over other three surfaces indicate positive contributions to both sensible and latent heat fluxes and govern their changes. The variations in the aerodynamic resistance and the surface resistance lead to opposite contributions. Besides, both radiative surface temperature and surface air temperature over the sparse surfaces are significantly higher than that over the closed shrubland. Larger aerodynamic resistance and Bowen ratio over the sparse vegetation dominate the warming trend accompanying the vegetation degradation and simultaneously offset the cooling effect induced by the decrease in net radiation, showing the land surface process over different surfaces factually has an evident feedback on surface micro-climate in the same climate and weather background.

Performance of One-Dimensional Multilayer Nanostructure Emitter and Filter for Thermophotovoltaic Applications

In this paper, a one - dimensional multilayer is optimized for potential applications as thermophotovoltaic (TPV) selective emitter and filter. Both of the proposed structures were prepared through a magnetron sputtering process. The influence of multilayer selective emitter and filter on the radiative properties is studied by using the rigorous coupled-wave analysis (RCWA). The emittance spectrum of the proposed TPV selective emitter shows three emittance peaks which are explained by the surface plasmon polariton (SPP), gap Plasmon polariton (GPP) and magnetic polariton (MP) excitation. The results show that the proposed emitter has high net radiation power. The calculated results show that the proposed filter has high transmittance for photons at

Inter‐annual variability in the effects of riparian woodland on micro‐climate, energy exchanges and water temperature of an upland Scottish stream

AbstractThe influence of riparian woodland on stream temperature, micro‐climate and energy exchange was investigated over seven calendar years. Continuous data were collected from two reaches of the Girnock Burn (a tributary of the Aberdeenshire Dee, Scotland) with contrasting land use characteristics: (1) semi‐natural riparian forest and (2) open moorland. In the moorland reach, wind speed and energy fluxes (especially net radiation, latent heat and sensible heat) varied considerably between years because of variable riparian micro‐climate coupled strongly to prevailing meteorological conditions. In the forested reach, riparian vegetation sheltered the stream from meteorological conditions that produced a moderated micro‐climate and thus energy exchange conditions, which were relatively stable between years. Net energy gains (losses) in spring and summer (autumn and winter) were typically greater in the moorland than the forest. However, when particularly high latent heat loss or low net radiation gain occurred in the moorland, net energy gain (loss) was less than that in the forest during the spring and summer (autumn and winter) months. Spring and summer water temperature was typically cooler in the forest and characterised by less inter‐annual variability due to reduced, more inter‐annually stable energy gain in the forested reach. The effect of riparian vegetation on autumn and winter water temperature dynamics was less clear because of the confounding effects of reach‐scale inflows of thermally stable groundwater in the moorland reach, which strongly influenced the local heat budget. These findings provide new insights as to the hydrometeorological conditions under which semi‐natural riparian forest may be effective in mitigating river thermal variability, notably peaks, under present and future climates. © 2014 The Authors. Hydrological Processes published by John Wiley & Sons Ltd.

Evapotranspiration and Transpiration Coupling to the Atmosphere of Sugarcane in Southern Brazil: Scaling Up from Leaf to Field

In some of the traditional areas growing sugarcane and in most of the expanding regions in Brazil, water deficit stress is a limiting factor and irrigation is usually needed to assure economically viable sugarcane yields. This research evaluated the water requirements of a drip-irrigated second ratoon sugarcane crop based on three different spatial scales: field, plant and leaf. The Bowen ratio method (BRM) was used to evaluate the mass and energy exchanges over the field, further computing the crop evapotranspiration (ETc). Sap flow by heat balance method installed in four representative stalks was used to evaluate the water use at the plant scale, and an infra-red gas analyser was used to evaluate leaf transpiration and stomatal conductance at the leaf scale. Comparing ETc and reference evapotranspiration we found that the crop coefficients for dry and wet seasons were 0.81 and 0.98, respectively. Evapotranspiration peaks of 7 mm day−1 were observed under conditions of strong crop-atmosphere coupling and high net radiation values. The stomatal conductance ranged from 0.09 to 0.19 mmol m−2 s−1 in dry and wet seasons respectively, showing sensitivity to the atmospheric conditions, notably the vapour deficit pressure and wind speed. BRM ranged from 3.7 to 4.4 mm day−1 and leaf transpiration ranged among 3.4 and 4.2 mm day−1. In an irrigated sugarcane production area in Brazil, water consumption of the sugarcane crop was highly correlated with atmospheric conditions.

Change in snow phenology and its potential feedback to temperature in the Northern Hemisphere over the last three decades

Trends in the duration or extent of snow cover are expected to feedback to temperature trends. We analyzed trends in dates of onset and termination of snow cover in relation to temperature over the past 27 years (1980–2006) from over 636 meteorological stations in the Northern Hemisphere. Different trends in snow duration are observed over North America and Eurasia. Over North America, the termination date of snow cover remained stable during the 27 years, whereas over Eurasia it has advanced by 2.6 ± 5.6 d decade−1. Earlier snow cover termination is systematically correlated on a year-to-year basis with a positive temperature anomaly during the snowmelt month with a sensitivity of −0.077 °C d−1. These snow feedbacks to air temperature are more important in spring, because high net radiation is coupled with thin snow cover.

Radiation and water balance properties of cadmium-polluted maize in a wet year

The effect of spraying maize weekly with a 0.5 M concentration of Cd was studied in Hungary at two water supply levels. Cd reduced the size of the leaf area and the annual water loss. The intensity of evapotranspiration (mm water/unit leaf area) of the polluted crops grew. The higher net radiation of polluted maize resulted in an increase in sensible heat and a warmer canopy. The warmer the canopy, the higher the stress level of the contaminated crops was. While Cd caused a 28% significant decline in plant dry matter production in the rainfed plots, only a declining tendency amounting to a few percent was recorded in the ET treatment.Cd of atmospheric origin was only detected in the leaves; it was not accumulated in the grain. It was concluded from the results that the yield losses in grain maize grown on Cd-polluted areas could be mitigated by irrigation. The production of silage maize, however, is not recommended in regions affected by Cd pollution, especially in areas where irrigation is required, since supplementary water supplies promote the accumulation of Cd in the vegetative organs.

An analysis of snow cover changes in the Himalayan region using MODIS snow products and in-situ temperature data

Amidst growing concerns over the melting of the Himalayas’ snow and glaciers, we strive to answer some of the questions related to snow cover changes in the Himalayan region covering Nepal and its vicinity using Moderate Resolution Imaging Spectroradiometer (MODIS) snow cover products from 2000 to 2008 as well as in-situ temperature data from two high altitude stations and net radiation and wind speed data from one station. The analysis consists of trend analysis based on the Spearman’s rank correlation on monthly, seasonal and annual snow cover changes over five different elevation zones above 3,000 m. There are decreasing trends in January and in winter for three of the five elevation zones (all below 6,000 m), increasing trends in March for two elevation zones above 5,000 m and increasing trends in autumn for four of the five elevation zones (all above 4,000 m). Some of these observed trends, if continue, may result in changes in the spring and autumn season river flows in the region. Dominantly negative correlations are observed between the monthly snow cover and the in-situ temperature, net radiation and wind speed from the Pyramid station at 5,035 m (near Mount Everest). Similar correlations are also observed between the snow cover and the in-situ temperature from the Langtang station at 3,920 m elevation. These correlations explain some of the observed trends and substantiate the reliability of the MODIS snow cover products.

Modeling Potential Evapotranspiration of Two Forested Watersheds in the Southern Appalachians

Global climate change has direct impacts on watershed hydrology through altering evapotranspiration (ET) processes at multiple scales. There are many methods to estimate forest ET with models, but the most practical and the most popular one is the potential ET (PET) based method. However, the choice of PET methods for AET estimation remains challenging. This study explored ways to identify appropriate PET models for two small forested watersheds, one dominated by conifer plantation and one dominated by native naturally regenerated deciduous hardwoods, by using long-term hydrometeorological data collected at the Coweeta Hydrologic Laboratory in the humid Appalachians in the southeastern U.S. Our specific objectives were to: (1) contrast three common PET models (FAO-56 grass reference ET, Hamon PET, and Priestley-Taylor PET) and compare these PET estimates with measured AET at monthly and annual temporal scales, and (2)�derive correction factors for the FAO-56 grass reference ET and Hamon PET models at the monthly scale using the Priestley-Taylor equation as the standard method for estimating forest PET. We found that different PET models gave significantly different PET estimates. The Priestley-Taylor equation gave the most reasonable estimates of forest PET for both watersheds. We conclude that the uncorrected Hamon and FAO PET methods would cause large underestimates of forest PET. Annual PET rates of the conifer watershed were higher than those of the native deciduous watershed due to the lower albedo (thus higher net radiation) in the former compared to the latter. Monthly correction factors provided useful tools for forest PET estimation in those areas lacking climatic data (i.e., radiation, humidity, and wind speed).

Some practical notes on the land surface modeling in the Tibetan Plateau

Abstract. The Tibetan Plateau is a key region of land-atmosphere interactions, as it provides an elevated heat source to the middle-troposphere. The Plateau surfaces are typically characterized by alpine meadows and grasslands in the central and eastern part while by alpine deserts in the western part. This study evaluates performance of three state-of-the-art land surface models (LSMs) for the Plateau typical land surfaces. The LSMs of interest are SiB2 (the Simple Biosphere), CoLM (Common Land Model), and Noah. They are run at typical alpine meadow sites in the central Plateau and typical alpine desert sites in the western Plateau. The identified key processes and modeling issues are as follows. First, soil stratification is a typical phenomenon beneath the alpine meadows, with dense roots and soil organic matters within the topsoil, and it controls the profile of soil moisture in the central and eastern Plateau; all models, when using default parameters, significantly under-estimate the soil moisture within the topsoil. Second, a soil surface resistance controls the surface evaporation from the alpine deserts but it has not been reasonably modeled in LSMs; an advanced scheme for soil water flow is implemented in a LSM, based on which the soil resistance is determined from soil water content and meteorological conditions. Third, an excess resistance controls sensible heat fluxes from dry bare-soil or sparsely vegetated surfaces, and all LSMs significantly under-predict the ground-air temperature gradient, which would result in higher net radiation, lower soil heat fluxes and thus higher sensible heat fluxes in the models. A parameterization scheme for this resistance has been shown to be effective to remove these biases.

Evaluating the Crop Water Stress Index and its correlation with latent heat and CO2 fluxes over winter wheat and maize in the North China plain

Plant water status is a key factor impacting crop growth and agricultural water management. Crop water stress may alter canopy temperature, the energy balance, transpiration, photosynthesis, canopy water use efficiency, and crop yield. The objective of this study was to calculate the Crop Water Stress Index (CWSI) from canopy temperature and energy balance measurements and evaluate the utility of CWSI to quantify water stress by comparing CWSI to latent heat and carbon dioxide (CO(2)) flux measurements over canopies of winter wheat (Triticum aestivum L) and summer maize (Zea mays L.). The experiment was conducted at the Yucheng Integrated Agricultural Experimental Station of the Chinese Academy of Sciences from 2003 to 2005. Latent heat and CO(2) fluxes (by eddy covariance), canopy and air temperature, relative humidity, net radiation, wind speed, and soil heat flux were averaged at half-hour intervals. Leaf area index and crop height were measured every 7 days. CWSI was calculated from measured canopy-air temperature differences using the Jackson method. Under high net radiation conditions (greater than 500 W m(-2)), calculated values of minimum canopy-air temperature differences were similar to previously published empirically determined non-water-stressed baselines. Valid measures of CWSI were only obtained when canopy closure minimized the influence of viewed soil on infrared canopy temperature measurements (leaf area index was greater than 2.5 m(2) m(-2)). Wheat and maize latent heat flux and canopy CO(2) flux generally decreased linearly with increases in CWSI when net radiation levels were greater than 300 W m(-2). The responses of latent heat flux and CO(2) flux to CWSI did not demonstrate a consistent relationship in wheat that would recommend it as a reliable water stress quantification tool. The responses of latent heat flux and CO(2) flux to CWSI were more consistent in maize, suggesting that CWSI could be useful in identifying and quantifying water stress conditions when net radiation was greater than 300 W m(-2). The results suggest that CWSI calculated by the Jackson method under varying solar radiation and wind speed conditions may be used for irrigation scheduling and agricultural water management of maize in irrigated agricultural regions, such as the North China Plain. Published by Elsevier B.V.