Forest Evapotranspiration Research Articles

The influence of water table depth on evapotranspiration in the Amazon arc of deforestation

Abstract. The Amazon rainforest evapotranspiration (ET) flux provides climate-regulating and moisture-provisioning ecosystem services through a moisture recycling system. The dense complex canopy and deep root system creates an optimum structure to provide large ET fluxes to the atmosphere, forming the source of precipitation. Extensive land use and land cover change (LULCC) from forest to agriculture in the arc of deforestation breaks this moisture recycling system. Crops such as soybean are planted in large homogeneous monocultures and the maximum rooting depth of these crops is far shallower than forest. This difference in rooting depth is key as forests can access deep soil moisture and show no signs of water stress during the dry season, while in contrast crops are highly seasonal with a growing season dependent on rainfall. As access to soil moisture is a limiting factor in vegetation growth, we hypothesised that if crops could access soil moisture, they would undergo less water stress and therefore would have higher evapotranspiration rates than crops which could not access soil moisture. We combined remote-sensing data with modelled groundwater table depth (WTD) to assess whether vegetation in areas with a shallow WTD had higher ET than vegetation in deep WTD areas. We randomly selected areas of forest, savanna, and crop with deep and shallow WTD and examined whether they differ on MODIS Evapotranspiration (ET), Land Surface Temperature (LST), and Enhanced Vegetation Index (EVI), from 2001 to 2012, annually and during transition periods between the wet and dry seasons. As expected, we found no differences in ET, LST, and EVI for forest vegetation between deep and shallow WTD, which because of their deep roots could access water and maintain evapotranspiration for moisture recycling during the entire year. We found significantly higher ET and lower LST in shallow WTD crop areas than in deep WTD during the dry season transition, suggesting that crops in deep WTD undergo higher water stress than crops in shallow WTD areas. The differences found between crop in deep and shallow WTD, however, are of low significance with regards to the moisture recycling system, as the difference resulting from conversion of forest to crop has an overwhelming influence (ET in forest is ≈2 mm d−1 higher than that in crops) and has the strongest impact on energy balance and ET. However, access to water during the transition between wet and dry seasons may positively influence growing season length in crop areas.

Modeling boreal forest evapotranspiration and water balance at stand and catchment scales: a spatial approach

Abstract. Vegetation is known to have strong influence on evapotranspiration (ET), a major component of terrestrial water balance. Yet hydrological models often describe ET by methods unable to include the variability of vegetation characteristics in their predictions. To take advantage of the increasing availability of high-resolution open GIS data on land use, vegetation and soil characteristics in the boreal zone, a modular, spatially distributed model for predicting ET and other hydrological processes from grid cell to catchment level is presented and validated. An improved approach to upscale stomatal conductance to canopy scale using information on plant type (conifer/deciduous) and stand leaf-area index (LAI) is proposed by coupling a common leaf-scale stomatal conductance model with a simple canopy radiation transfer scheme. Further, a generic parametrization for vegetation-related hydrological processes for Nordic boreal forests is derived based on literature and data from a boreal FluxNet site. With the generic parametrization, the model was shown to reproduce daily ET measured using an eddy-covariance technique well at 10 conifer-dominated Nordic forests whose LAI ranged from 0.2 to 6.8 m2 m−2. Topography, soil and vegetation properties at 21 small boreal headwater catchments in Finland were derived from open GIS data at 16 m × 16 m grid size to upscale water balance from stand to catchment level. The predictions of annual ET and specific discharge were successful in all catchments, located from 60 to 68∘ N, and daily discharge was also reasonably well predicted by calibrating only one parameter against discharge measurements. The role of vegetation heterogeneity in soil moisture and partitioning of ET was demonstrated. The proposed framework can support, for example, forest trafficability forecasting and predicting impacts of climate change and forest management on stand and catchment water balance. With appropriate parametrization it can be generalized outside the boreal coniferous forests.

Surface conductance for evapotranspiration of tropical forests: Calculations, variations, and controls

Tropical forests are responsible for the evaporation and transpiration of large quantities of water into the atmosphere annually. Surface conductance (gs) is a poorly understood phenomenon that plays a central role in regulating this evapotranspiration. We studied the calculations, variations, and environmental factors controlling gs based on eddy flux measurements from 10 tropical forest sites that covered a wide range of water gradients across continents. We found that boundary layer conductance (gb) is comparable with aerodynamic conductance for momentum (gaM) and thus, it should not be ignored in estimations of total aerodynamic conductance for water vapor (gaV). Based on the findings, we have made some recommendations for gaM estimation both with and without measurements of turbulence. The seasonal variation of gs is low in humid sites but large in sites with a dry season. A value of 24.8 ± 13.8 mm s−1 was suggested for maximum surface conductance (gsmax) for tropical forests. Both water vapor deficit (D) and radiation (Q) play an important role in controlling gs. The model driven by both D and Q could capture the diurnal variations of gs well and could be implemented in large-scale models in future. We believe the findings of this study could contribute substantially to our understanding of tropical forest gs.

Impacts of forestation and deforestation on local temperature across the globe.

Changing forest cover is a key driver of local climate change worldwide, as it affects both albedo and evapotranspiration (ET). Deforestation and forestation are predicted to have opposing influences on surface albedo and ET rates, and thus impact local surface temperatures differently. Relationships between forest change, albedo, ET, and local temperatures may further vary regionally, as the strengths of warming by albedo effects and cooling by ET effects vary with latitude. Despite these important relationships, the magnitude of forest cover effects on local surface temperature across the globe remains unclear. Using recently-released global forest change data, we first show that forestation and deforestation have pervasive and opposite effects on LST, ET and albedo worldwide. Deforestation from 2000 to 2010 caused consistent warming of 0.38 ± 0.02 (mean ± SE) and 0.16 ± 0.01°C in tropical and temperate regions respectively, while forestation caused cooling in those regions of -0.18 ± 0.02 and -0.19 ± 0.02°C. Tropical forests were particularly sensitive to the climate effects of forest change, with forest cover losses of ~50% associated with increased LST of 1.08 ± 0.25°C, whereas similar forest cover gains decreased LST by -1.11 ± 0.26°C. Secondly, based on a new structural equation model, we show that these changes on LST were largely mediated by changes in albedo and ET. Finally, based on this model, we show that predicted forest changes in Brazil associated with a business-as-usual land use scenario through 2050 may increase LST up to 1.45°C. Our results contribute to a better understanding of the mechanistic inter-relationships between forest change and changes in albedo, ET and LST, and provide additional evidence that forestation has the potential to reverse deforestation impacts on local climate, especially in tropical and temperate regions.

Water Yield Responses to Gradual Changes in Forest Structure and Species Composition in a Subboreal Watershed in Northeastern China

Relationships between forest cover and streamflow have been studied worldwide, but only a few studies have examined how gradual changes in forest structure and species composition due to logging and climate change affect watershed water yield (Q) and flow regimes. In this study, we analyzed long-term (45 years) hydrologic, climate and forest dynamics data from the subboreal Tahe watershed in northeastern China. Our purpose was to evaluate the effects of forest logging and regeneration on changes in forest biomass and species and to quantify the subsequent impact on mean annual streamflow and flow regime under a changing climate. The study watershed was dominated by old-growth larch (Larix gmelinii Rupr.) during the 1970s, but gradually transformed into young deciduous larch mixed with deciduous broad-leaved birch (Betula platyphylla Sukaczev) during the 2010s. During the same period, the watershed experienced climate change with a significant increase in air temperature of 0.028 ℃/year. We applied eight sensitivity-based techniques to separate the effects of climate change on water yield from those due to forest changes. We used flow duration curves (FDCs) to characterize flow regimes by dividing the study into four key periods based on the proportional change of larch and birch trees. We found that the mean annual streamflow decreased by 10 mm (−16 mm attributed to forest change and +6 mm to climate change) between the 1984–1994 period and the 2006–2016 period when the proportion of birch increased by 20% with a similar total forest volume in the later period. The mean annual streamflow increased from 216 mm to 270 mm (+35.5 mm due to forest change vs +17.7 mm due to climate change) when forest volume decreased by 18.7% (17 m3/ha) between the 1970s and 1984–1994. Water yield changed only slightly (3.5 mm) when forest volume increased by 8.7% (6 m3/ha) from 2000 to 2011. In addition, the magnitude of high flow and low flow increased following deforestation and a shift in species composition from a period (1984–1994) with 70% larch with 30% birch to a later period (2006–2016) with 50% larch with 50% birch. Both high flow and low flow decreased coinciding with a reforestation period (2006–2016). Our results highlight complex interactions among climate, forest structure, total biomass, and plant diversity (trees species composition) in influencing watershed hydrology. Further study is needed to examine the effects of ecohydrological processes such as evapotranspiration in larch and birch forests on hydrologic changes across multiple scales.

Carbon Dioxide and Water Exchange between Spruce Forest and Atmosphere in Spring—Summer under Different Weather Conditions

Results of eddy covariance measurements of carbon dioxide and water fluxes above a spruce forest in the middle taiga subzone from April to August 2013 and 2016 are presented. The ecosystem of the spruce forest turned from source to sink CO2 at mean daily air temperatures below zero in late March and early April; in 2016, it was 2 weeks earlier than in 2013. The maximum net ecosystem exchange of CO2 (NEE) was recorded at the end of June and beginning of July. In 2016, the mean daily NEE of spruce forest decreased at a high air temperature and a low amount of precipitation in the beginning of the growing season; turning from the sink CO2 to the source was observed in early August. The net exchange of CO2 between spruce and the atmosphere from April to August 2013 reached 327 g C/m2 and, in 2016, 174 g C/m2. The seasonal evapotranspiration of the spruce forest in these years was 239–247 mm/m2 and the mean value of water-use efficiency for photosynthesis (WUE) in the season was 2.3–3.3 g C/kg H2O. WUE was relatively constant during the growing season, resulting from a close relationship between the exchange of CO2 and water, the main processes that ensure the production of organic matter in the spruce-forest ecosystem.

三北工程黄土高原丘陵沟壑区森林降温增湿效果研究

PDF HTML阅读 XML下载 导出引用 引用提醒 三北工程黄土高原丘陵沟壑区森林降温增湿效果研究 DOI: 10.5846/stxb201812282830 作者: 作者单位: 中国科学院地理科学与资源研究所,中国科学院地理科学与资源研究所,中国科学院地理科学与资源研究所,北京林业大学自然保护区学院,中国科学院地理科学与资源研究所 作者简介: 通讯作者: 中图分类号: 基金项目: 国家重点研发计划课题（2016YFC0503706，2016YFC0503403）；中国科学院科技服务网络计划项目资助（KFJ-STS-RKT-005）；中国科学院A类战略性先导科技专项资助（XDA20020402） The cooling and humidifying effect by the forest ecosystem in the hilly and gully area of Loess Plateau of the Three North Shelter Forest System Project region Author: Affiliation: Institute of Geographical Sciences and Natural Resources Research,Institute of Geographical Sciences and Natural Resources Research,Institute of Geographical Sciences and Natural Resources Research,School of Conservation, Beijing Forestry University,Institute of Geographical Sciences and Natural Resources Research Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:生态系统可通过植被蒸腾与土壤蒸发作用调节区域温度与湿度。三北工程黄土高原丘陵沟壑区属于干旱和半干旱区，森林降温增湿功能有助于改善区域生存环境。在分析研究区森林覆被变化基础上，利用Penman-Monteith公式模拟了森林实际蒸散量，研究了森林增湿与降温效果。研究结果显示：（1）1980-2015年研究区森林面积增加了2.25%，主要来自荒草地、耕地和荒漠；（2）1980-2015年研究区森林6-9月实际蒸散总量为1.19×1010-1.40×1010 t/a，平均实际蒸散量为219-257 mm，可使区域6-9月绝对湿度每日增加0.47-0.55 g/m3，相对湿度每日增加2.87%-3.32%；（3）森林通过蒸散作用吸热量为29.15×1015-34.26×1015 kJ/a，单位面积蒸散吸热量为53.72×108-63.13×108 kJ hm-2 a-1，通过蒸散吸热日降温量为0.92-1.08℃/d；（4）研究区森林蒸散量在1980-2010年逐渐增加，但在2015年明显下降，这主要是由降水减少导致；森林面积较大的山西和陕西森林蒸散降温增湿效果较好。通过对比相同年份不同土地覆被发现，森林实际蒸散量显著高于其他土地覆被类型。因此，未来研究区可在水资源承载能力范围内适当增加森林面积，充分发挥森林调节区域气候的作用。 Abstract:The ecosystem can regulate regional temperature and humidity through evapotranspiration. The climate in the hilly and gully area of the Loess Plateau in the Three North Shelter Forest System Project (TNSFSP) region is arid and semi-arid, and the cooling and humidifying function of the forest ecosystem is crucial for improving the living environment in this region. Based on the analysis of forest cover changes in this area, in this study, we simulated using the Penman-Monteith formula and investigated the contribution of evapotranspiration by ecosystems to regional climate regulation. The results show that:(1) The forest area in the study region (mainly grassland, cropland, and desert) increased by 2.25% from 1980 to 2015. (2) The total amount of forest actual evapotranspiration (AET) in the study area from 1980 to 2015 was between 1.19×1010 t/a and 1.40×1010 t/a, and average AET ranged from 219 mm to 257 mm. Absolute humidity from June to September in 1980 to 2015 increased by 0.47-0.55 g/m3 per day, and the relative humidity will increase by 2.87%-3.32% per day. (3) Heat absorption of the forest through evapotranspiration was estimated as 29.15×1015 to 34.26×1015 kJ/a, evapotranspiration per hectare was 53.72×108-63.13×108 kJ hm-2 a-1), and the daily cooling rate by evapotranspiration was 0.92-1.08℃ d-1. (4) The amount of forest evapotranspiration increased gradually from 1980 to 2010, but it decreased significantly in 2015, which was mainly caused by the decrease in precipitation. Spatially, forests in Shanxi and Shaanxi provinces, with a larger forest area, had better evapotranspiration and humidification effects. Comparison of the different land cover in 1980 to 2015 showed that the actual evapotranspiration of the forest was still significantly higher than other land cover types. In future, the forest area will be appropriately increased within the carrying capacity of water resources, and the role of forest will be even more crucial in regional climate regulation. 参考文献 相似文献 引证文献

Automated Geospatial Models of Varying Complexities for Pine Forest Evapotranspiration Estimation with Advanced Data Mining

The study goal was to develop automated user-friendly remote-sensing based evapotranspiration (ET) estimation tools: (i) artificial neural network (ANN) based models, (ii) ArcGIS-based automated geospatial model, and (iii) executable software to predict pine forest daily ET flux on a pixel- or plot average-scale. Study site has had long-term eddy-flux towers for ET measurements since 2006. Cloud-free Landsat images of 2006−2014 were processed using advanced data mining to obtain Principal Component bands to correlate with ET data. The regression model’s r2 was 0.58. The backpropagation neural network (BPNN) and radial basis function network (RBFN) models provided a testing/validation average absolute error of 0.18 and 0.15 Wm−2 and average accuracy of 81% and 85%, respectively. ANN models though robust, require special ANN software and skill to operate; therefore, automated geospatial model (toolbox) was developed on ArcGIS ModelBuilder as user-friendly alternative. ET flux map developed with model tool provided consistent ET patterns for landuses. The software was developed for lay-users for ET estimation.

Carbon dioxide, water vapour and energy fluxes over a semi-evergreen forest in Assam, Northeast India

The eddy covariance method is a powerful technique for quantification of \(\hbox {CO}_{2},\) \(\hbox {H}_{2}\)O and energy fluxes in natural ecosystems. Leaf area index (LAI) and its changes are significant drivers of \(\hbox {CO}_{2}\) and \(\hbox {H}_{2}\)O exchange in a forest ecosystem due to their role in photosynthesis. The present study reports the seasonal variation of \(\hbox {CO}_{2}\) and energy fluxes and their relationship with other meteorological parameters of a semi-evergreen primary forest of Kaziranga National Park, Assam, India during February 2016–January 2017. The diurnal pattern of half hourly average \(\hbox {CO}_{2 }\) fluxes over the forest was found to be mostly dominated by the incident photosynthetically active radiation. During the period of study, diurnal variations of \(\hbox {CO}_{2}\) flux showed a maximum value of \(-9.97\,\upmu \)mol \(\hbox {m}^{-2}\hbox {s}^{-1}\) in the month of June during summer which is also the beginning of the monsoon season. The monthly averaged diurnal \(\hbox {CO}_{2}\) flux and variation in LAI of the forest canopy closely followed each other. The annual net ecosystem exchange of the forest estimated from the \(\hbox {CO}_{2}\) flux data above the canopy is 84.21 g C \(\hbox {m}^{-2}\,\hbox {yr}^{-1}\). Further studies are in progress to confirm these findings. The estimated average annual evapotranspiration of the semi-evergreen forest is 2.8 ± 0.19 mm \(\hbox {day}^{-1}\). The study of partitioning of energy fluxes showed the dominance of latent heat fluxes over sensible heat fluxes. The energy balance closure was found to increase with an increase in instability and the highest closure of around 83% was noted under neutral conditions.

Modeling the Response of Daily Evapotranspiration and its Components of a Larch Plantation to the Variation of Weather, Soil Moisture, and Canopy Leaf Area Index

AbstractAn accurate prediction of forest evapotranspiration (ET) based on its components in response to a changing environment is essential for understanding interactions between the atmosphere, soil, and vegetation and for integrated forest‐water management. The ET components of a pure larch plantation and the environment were monitored over 2 years in northwest China. The response functions of each ET component to individual driving factors were determined using upper boundary lines, then coupled to form the ET component modules, fitted with measured data in 2016 (May–September), and validated with measured data in 2015 (June–September). Results showed that (1) the response of daily transpiration (T) to potential ET (ETref) followed a binomial equation, and the response of T to relative extractable water (REW) of the 0‐ to 60‐cm soil layer and canopy leaf area index (LAI) followed a saturated exponential growth function. The module was T = (−5.766 × 10−4ETref2 + 0.005ETref–0.002) × (18.769 + 46.990 (1–exp(−8.555REW))) × (−14.662 + 17.428 (1–exp(−1.414LAI))). (2) The response of daily forest floor evapotranspiration (FE) to ETref, volumetric soil moisture (VSM) of the 0‐ to 30‐cm soil layer, and LAI followed positive linear, saturated exponential growth and saturated exponential decay function. The module was FE = (6.697ETref–2.770) × (6.927–11.243exp(−1.959VSM)) × (0.032 + 1.162exp(−2.407LAI)). (3) The canopy interception (Ic) of individual rainfall events was mainly affected by precipitation amount (P) and LAI. The module was Ic = 0.446 LAI (1–exp(−0.112P)) + 0.097P. (4) The daily ET model was built up as ET = T + FE + Ic, which had good performance in both the calibration and validation period. It can be concluded that the variation of daily forest ET can be accurately predicted using the easily measurable driving factors of weather, soil, and vegetation.

Separating the impacts of climate change and human activities on actual evapotranspiration in Aksu River Basin ecosystems, Northwest China

Abstract Separating the impacts of climate change and human activity on actual evapotranspiration (ET) is important for reducing comprehensive risk and improving the adaptability of water resource systems. In this study, the spatiotemporal distribution of actual ET in the Aksu River Basin, Northwest China, during the period 2000–2015 was evaluated using the Vegetation Interfaces Processes model and Moderate Resolution Imaging Spectroradiometer-Normalized Difference Vegetation Index. The impact of climate change and human activity on actual ET were separated and quantified. The results demonstrated that: (1) the annual pattern of actual ET per pixel exhibited the highest values for arable land (average 362.4 mm/a/pixel), followed by forest land and grassland (average of 159.6 and 142.8 mm/a/pixel, respectively). Significant increasing linear trends (p < 0.05) of 3.2 and 1.8 mm/a were detected in the arable land and forest land time series, respectively; (2) precipitation was the most significant of the selected climate factors (precipitation, average temperature, sunshine duration, and wind speed) for all ecosystems. The second most significant was wind speed; (3) human activity caused 89%, 98%, and 80% of the changes in actual ET of forest, grass, and arable land, respectively, while climate change caused 11%, 2%, and 20% of the changes in actual ET, in the Aksu River Basin during 2000–2015.

Drought and thinning have limited impacts on evapotranspiration in a managed pine plantation on the southeastern United States coastal plain

Managed and natural coastal plain forests in the humid southeastern United States exchange large amounts of water and energy with the atmosphere through the evapotranspiration (ET) process. ET plays an important role in controlling regional hydrology, climate, and ecosystem productivity. However, long-term studies on the impacts of forest management and climatic variability on forest ET are rare, and our understanding of both external and internal drivers on seasonal and interannual ET variability is incomplete. Using techniques centered on an eddy covariance method, the present study measured year-round ET flux and associated hydrometeorological variables in a drained loblolly pine (Pinus taeda L.) plantation on the lower coastal plain of North Carolina, U.S. We found that annual ET was relatively stable (1076 ± 104 mm) in comparison to precipitation (P) (1168 ± 216 mm) during the 10-year study period when the site experienced extreme climate (2007–2008) and forest thinning (2009). At the seasonal time scale, mean ET/P varied between 0.41 and 1.51, with a mean value of 1.12 ± 0.23 and 0.72 ± 0.16 for the growing and dormant seasons, respectively. The extreme drought during 2007–2008 (mean annual P, 854 mm) only resulted in a slight decrease (∼8%) in annual ET owing to the shallow groundwater common to the study area. Although changes in leaf area index and canopy structure were large after the stand was 50% thinned in the fall of 2009, mean annual ET was similar and averaged 1055 mm and 1104 mm before (2005, 2006 and 2009) and after (2010–2015) thinning, respectively. Data suggested that annual ET recovered within two years of the thinning as a result of rapid canopy closure and growth of understory. Further analysis indicated that available energy was the key driver of ET: approximately 69% and 61% of the monthly variations in ET were explained by net radiation during the dormant and growing seasons, respectively. Overall, we concluded that drought and forest thinning had limited impacts on seasonal and annual ET in this energy limited forest ecosystem with shallow groundwater. The results from this study help to better understand regional ecohydrological processes and projecting potential effects of forest management and extreme climate on water and carbon cycles.

Enhancing SWAT simulation of forest ecosystems for water resource assessment: A case study in the St. Croix River basin

Forests often cover a significant portion of a watershed and play an important role in water, carbon, and nutrient cycling. The widely-used Soil and Water Assessment Tool (SWAT) has not been thoroughly tested for simulating forest ecosystems and associated impacts on watershed-scale water resource assessment. Using the St. Croix River basin as a testbed, we examined SWAT simulations of forest biomass, net primary productivity (NPP), evapotranspiration (ET), and water yield with default and improved model settings. Model simulations with the default parameterization significantly underestimated forest biomass, ET, and NPP relative to benchmark data. With adjusted forest growth and development parameters, including radiation use efficiency, temperature control on plant growth, and leaf to biomass fraction, as well as a new phosphorus supply module, the SWAT model’s performance with respect to the above variables was substantially improved. These improvements also enhanced SWAT estimates of water yield, streamflow, and sediment loads at the watershed scale. Results of this study show the important role of forest ecosystems in watershed scale water balance and the connections between terrestrial and aquatic processes via lateral water and sediment fluxes. We suggested that watershed models, such as SWAT, should be assessed for both terrestrial and aquatic processes in order to provide credible water quantity and quality simulations at the watershed scale. Calibrated forest parameters and the phosphorus input algorithms will be applicable to future SWAT simulations of watershed hydrology.

Temporal evaluation of evapotranspiration for sugar cane, planted forest and native forest using landsat 8 images and a two-source energy balance

It is known that vegetation needs a certain amount of water to grow and develop. However, it can be difficult to know the actual water requirement of a specific crop. To quantify the hydric balance in hydrographic basins, detailed knowledge of the components of the hydrological cycle is necessary, especially regarding evapotranspiration (ET). Knowledge of ET of crops and forests, in general, is important at all stages of production management of the vegetation cover. Many studies have been developed with the aim of obtaining these values spatially, but temporal variability remains largely uncertain. Thus, the present study had the objective of utilizing a hybrid Two-Source Energy Balance (TSEB), adjusted to integrate hourly, monthly and yearly ET data, aiming at obtaining ET for two years of study, for areas with sugar cane, planted forest and native forest. How evapotranspiration behaves temporally could then be evaluated. Therefore, after analyzing monthly, seasonal and annual results it could be concluded that sugar cane consumes less water than planted forest and native forest with the same rainfall and environment for all uses.

Estimating evapotranspiration change due to forest treatment and fire at the basin scale in the Sierra Nevada, California

AbstractWe investigated the potential magnitude and duration of forest evapotranspiration (ET) decreases resulting from forest‐thinning treatments and wildfire in west‐slope watersheds of the Sierra Nevada range in California, USA, using a robust empirical relation between Landsat‐derived mean‐annual normalized difference vegetation index (NDVI) and ET measured at flux towers. Among forest treatments, the minimum observed NDVI change required to produce a significant departure from control plots with NDVI of about 0.70 was −0.09 units, corresponding to a basal‐area reduction of 29.1 m2/ha (45% reduction) and equivalent to an estimated ET reduction of 153 mm/year (21% change; approximate mean annual precipitation = 1,000 mm). Intensive thinning in highly productive forests that approached prefire‐exclusion densities reduced basal area by 40–50%, generating estimated ET reductions of 153–218 mm/year (21–27% change) over 5 years following treatment. Low‐intensity underburn treatments resulted in no significant change in ET. Examining the cumulative impact of wildfires on ET between 1990 and 2008, we found that the lower and wetter American River basin (5,310 km2) generated more than twice the ET reduction per unit area than those in the higher and drier Kings River basin (4,790 km2), corresponding to greater water and energy limitations in the latter and greater fire severity in the former. A rough extrapolation of these results to the entire American River watershed suggests that ET reductions due to forest thinning by wildfire could approach 10% of full natural flows for dry years and 5% over all years.

Soil moisture response to white ash mortality following emerald ash borer invasion

Emerald ash borer (EAB) (Agrilus planipennis Fairmaire), an invasive forest insect first identified in southeastern Michigan in 2002, is established in at least 32 US states and three Canadian provinces. Ash (Fraxinus spp.) mortality rates in some forested areas exceed 90%, but to date, little is known about the potential effects of EAB-caused ash mortality on hydrological processes. More broadly, there is a need for information on the timing and magnitude of soil moisture response to species-specific mortality of overstory vegetation in deciduous forest systems. Soil moisture was examined in 28 forested sites where 0–100% of the white ash basal area (Fraxinus americana L.) was killed by EAB. Synoptic measurements of near-surface (0–6 cm depth) soil moisture were collected from 112 plots (18 m radius) within the sites. Three plots were also instrumented with soil moisture sensors at 10 and 25 cm depth to log hourly measurements from May to October. Synoptic data showing white ash mortality and soil moisture were positively correlated in the 34 plots with ≥ 5% mortality (by total basal area). In the intensively monitored plots, volumetric soil moisture declined from 37 to 16% between July and September where white ash mortality was low (0.6% of basal area killed), but remained near field capacity (~ 30%) throughout the monitoring period in the high mortality plot (8.6% of basal area killed), meriting further investigation to assess effects of white ash mortality on evapotranspiration and soil moisture dynamics in heterogeneous upland forests. Altered soil moisture may have implications for regrowth dynamics, infiltration/runoff partitioning, and nutrient cycling, but additional study to quantify the extent and duration of EAB-related ash mortality on hydrology at the plot and watershed scale is necessary.

Management Implications of Snowpack Sensitivity to Temperature and Atmospheric Moisture Changes in Yosemite National Park, CA

AbstractIn order to investigate snowpack sensitivity to temperature increases and end‐member atmospheric moisture conditions, we applied a well‐constrained energy‐ and mass‐balance snow model across the full elevation range of seasonal snowpack using forcing data from recent wet and dry years. Humidity scenarios examined were constant relative humidity (high) and constant vapor pressure between storms (low). With minimum calibration, model results captured the observed magnitude and timing of snowmelt. April 1 snow water equivalent (SWE) losses of 38%, 73%, and 90% with temperature increases of 2, 4, and 6°C in a dry year centered on areas of greatest SWE accumulation. Each 2°C increment of warming also resulted in seasonal snowline moving upslope by 300 m. The zone of maximum melt was compressed upward 100–500 m with 6°C warming, with the range reflecting differences in basin hypsometry. Melt contribution by elevations below 2,000 m disappeared with 4°C warming. The constant‐relative‐humidity scenario resulted in 0–100 mm less snowpack in late spring vs. the constant‐vapor‐pressure scenario in a wet year, a difference driven by increased thermal radiation (+1.2 W/m2) and turbulent energy fluxes (+1.2 W/m2) to the snowpack for the constant‐relative‐humidity case. Loss of snowpack storage and potential increases in forest evapotranspiration due to warming will result in a substantial shift in forest water balance and present major challenges to land management in this mountainous region.

Assessment of the indirect impact of wildfire (severity) on actual evapotranspiration in eucalyptus forest based on the surface energy balance estimated from remote-sensing techniques

ABSTRACTWildfires have a strong impact on the environment, changing its structure, soil properties, and microclimate and subsequently its water cycle with implications on the surface energy fluxes. Persisting droughts and catastrophic forest fires initiated this case study of pure eucalyptus stands in north-central Portugal. Although many studies have investigated changes in actual evapotranspiration (ETa), surface energy flux patterns, and the related physical parameters, only a few concentrated on the fire-driven changes in pure eucalyptus stands in the Mediterranean climate. This study aims to understand the consequences of wildfires on the water cycle, namely the ETa, and the surface energy heat fluxes by applying a simplified two-source energy balance model in combination with medium-resolution imagery (Landsat 8). A total of 21 different burnt locations were evaluated, which burned between 2011 and 2013. Estimated surface energy fluxes and daily ETa were compared to nearby control sites (unburnt) during satellite overpass for the time after the fire (2013–2015). The fire scars were classified into their burn severity, using the differenced Normalized Burn Ratio. The absolute difference of ETa ( ETa) between unburnt and burnt locations was used to identify fire-driven changes in magnitude and its evolution over time. Our results show that for the unburnt stands, the contributions to the total latent heat flux were around 80% from the canopy and 20% from the soil, while for the burnt site the contributions were around 30% (canopy) and 70% (soil) shortly after the fire. Inter-annually, the difference in ETa increased during the rainy season, which was related to the epicormic shooting, the fast regrowth rate of foliage, and the abundance of water. Generally, smaller differences in ETa were related to the severity classification and stand properties (i.e. tree species and soil characteristics). Two to three years after the fire events, ETa became non-significant for all severity classes, leading to an impact on the total water cycle smaller in comparison to other post-fire studies.

Overstorey evapotranspiration in a seasonally dry Mediterranean eucalypt forest: Response to groundwater and mining

AbstractGroundwater levels in the northern jarrah forest have declined at rates up to 0.5 m year−1 owing to increased aridity in south‐western Australia in the last 40 years. The forest has also been mined and rehabilitated resulting in significant areas of postmining forest. We tested the impact of declining groundwater levels and mining on evapotranspiration by jarrah forest overstorey. We hypothesized that trees in jarrah forest are facultative phreatophytes (will use groundwater where available but are not reliant on it) and water use per unit overstorey leaf area index (Los) of postmining forest is the same as that of postharvest forest. We measured sapflow at 7 sites in the northern jarrah forest and measured rainfall interception by the canopy at 9 sites. Stemflow was measured at 3 sites. Shallow depth to groundwater was associated with a larger ratio of transpiration per unit leaf area (Eos/Los), but there was little difference in Eos/Los between postmining and postharvest jarrah forest. Eos/Los ranged from 250 ‐ 340 mm year−1 (m2 m−2)−1 at sites where depth to groundwater was >15 m but was up to 400–500 mm year−1 (m2 m−2)−1 at some sites with shallow groundwater. Based on relationships between transpiration, rainfall interception, and Los, it is possible to estimate overstorey evapotranspiration in jarrah forest from Los, especially if spatial layers are available for depth to groundwater. We conclude that jarrah forest is conservative in its water use and likely to be resilient to a drying climate. Management implications for the northern jarrah forest are briefly discussed.

The influence of groundwater and land cover change on evapotranspiration in the Amazon Rainforest transition zone

The Amazon rainforest's moisture recycling system provides water for rainfed agriculture, hydroelectric power generation and human consumption, and is, therefore, an important ecosystem service which is hugely important for the Brazilian economy. However, forest conversion to agriculture in the Amazon severally reduces the evapotranspiration (ET) flux, especially over varying groundwater levels. During the dry season, when precipitation is limiting, access to groundwater can help maintain ET rates. This strongly depends on the rooting depth of vegetation and groundwater depth. These effects on ET may lead to lowering atmospheric moisture, and in turn, less moisture available for downstream precipitation. Understanding how land cover changes impact the moisture recycling systems could significantly influence future decision making. The aim of this research is to improve our understanding of the influence of groundwater depth, land use change and their interaction with ET of forest and agricultural land cover. We used one of the most commonly utilised remote sensing data products from MODIS (MOD16 - 8-day ET at 500m resolution) to investigate the temporal and spatial patterns of land cover change and evapotranspiration in the Amazon region from 2002 to 2017. First, we investigated groundwater's influence on the seasonality of ET under stable conditions, i.e., over areas that did not undergo land cover change during the study period. We found differences in seasonality as well as differences between deep and shallow groundwater for agricultural land cover types - Rangeland and Cropland. Secondly, we examined periods of land-use transitions where forest has been converted to agriculture in order to investigate the effect of transition on ET. The loss of ET following land cover change was clear although the time until the new land cover's ET reflected that of a stable land cover was highly variable. Lastly, areas of forest gain were examined to assess ET of secondary forest and the length of time until recovery of this ecosystem service. This analysis showed that secondary forests take several years until ET reaches stable values but it does return to values within the range of those for primary forest. These results suggest that groundwater and land use changes interact in their effect on ET especially in the seasonality and the time to recover.