Wetland Evapotranspiration Research Articles

Lake Fluctuation Effectively Regulates Wetland Evapotranspiration: A Case Study of the Largest Freshwater Lake in China

Lakes and wetlands provide valuable water resources. Wetland evapotranspiration (ET) is a key hydrologic component; however, the effects of lake fluctuation on wetland ET remain unclear. The Poyang Lake is the largest freshwater lake in China and experiences a dramatic fluctuation in water level and inundated area. This study used remote sensing data to estimate the wetland ET for Poyang Lake and to illustrate the distribution of wetland ET and its response to lake fluctuations. Our results showed that wetland ET was related to lake fluctuation both spatially and temporally. Within the same year, the difference between annual water evaporation (Ewater) and wetland ET (ETwetland) was primarily attributed to lake fluctuation through its effects on inundated area and exposure days. A 1% increase in inundated area would result in a 7.87 ± 1.13 mm a−1 reduction in annual Ewater-to-ETwetland differences, and a 10-day elongation of exposure could lead to an 11.1 ± 1.6 mm a−1 increase in annual Ewater-to-ETwetland differences, on average. Inter-annually, the Ewater-to-ETwetland differences were attributed to the combined effects of atmospheric and environmental variables and lake fluctuation. The lake fluctuation contributed 73% to the inter-annual ET difference, followed by relative humidity (19%), net radiation (5%), and wind speed (4%). Overall, lake fluctuation effectively regulates wetland ET, and its effect should receive careful consideration in hydrological and water resources studies under the current changing climate.

The importance of wetlands in the energy balance of an agricultural landscape

Energy fluxes, including net radiation, latent heat flux and sensible heat flux were determined on clear days during the vegetative period in four types of land cover: wet meadow, pasture, arable field, and an artificial concrete surface. The average net radiation ranged between 123 W m−2 at the concrete surface and 164 W m−2 at the wet meadow. The mean maximum daytime latent heat ranged between 500 and 600 W m−2, which corresponds to an evapotranspiration rate of about 0.2 g m−2 s−1 under the prevailing conditions of the wet meadow. The results demonstrated that the wet meadow dissipated about 30 % more energy through evapotranspiration than the field or the pasture, and up to 70 % more energy than the concrete surface. The evaporative fraction indicated that more than 100 % of the energy released by the wet meadow was dissipated through evapotranspiration; this was attributed to local heat advection. Wetland evapotranspiration thus contributes significantly to the cooling of agricultural landscapes; the energy released can reach several 100 MW km−2. Wetland evapotranspiration has a double ‘air conditioning’ effect through which it equalises temperature differences: (1) surplus solar energy is bound into water vapour as latent heat; (2) The vapour moves towards cooler portions of the atmosphere where the energy is released. The air-conditioning effect of wetlands plays an important role in mitigating local climate extremes; this ecosystem service tends to be disregarded in relation to other better-known wetland functions such as nutrient retention and provision of high biodiversity.

Effects of the introduction of rice on evapotranspiration in seasonal wetlands

AbstractLand use changes in wetland areas can alter evapotranspiration, a major component of the water balance, which eventually affects the water cycle and ecosystem. This study assessed the effect of introduced rice‐cropping on evapotranspiration in seasonal wetlands of northern Namibia. By using the Bowen ratio–energy balance method, measurements of evapotranspiration were performed over a period of 2.5 years at two wetland sites—a rice field (RF) and a natural vegetation field (NVF)—and at one upland field (UF) devoid of surface water. The mean evapotranspiration rates of RF (1.9 mm daytime−1) and NVF (1.8 mm daytime−1) were greater than that in UF (1.0 mm daytime−1). RF and NVF showed a slight difference in seasonal variations in evapotranspiration rates. During the dry season, RF evapotranspiration was less than the NVF evapotranspiration. The net radiation in RF was less in this period because of the higher albedo of the non‐vegetated surface after rice harvesting. In the early growth period of rice during the wet season, evapotranspiration in RF was higher than that in NVF, which was attributed to a difference in the evaporation efficiency and the transfer coefficient for latent heat that were both affected by leaf area index (LAI). Evapotranspiration sharply negatively responded to an increase in LAI when surface water is present according to sensitivity analysis, probably because a higher LAI over a surface suppresses evaporation. The control of LAI is therefore a key for reducing evaporation and conserving water. Copyright © 2013 John Wiley & Sons, Ltd.

Evapotranspiration affecting redox conditions in horizontal constructed wetlands under Mediterranean climate: Influence of plant species

The aim of this study was to conduct a comparative evaluation of evapotranspiration (ET) rates for eight different mesocosm constructed wetlands (CWs), and the relationship with redox potential (EH). Inflow, outflow and EH were measured over 4 years in winter and summer campaigns as well as over 24h on selected days in summer. Vegetation was the main design parameter which affected water loss in the wetlands (on average, ET in planted wetlands was 4 times higher than in unplanted ones), and Typha angustifolia was more active than Phragmites australis (mean daily ET – expressed as the average of ET rate measured every 2h in selected days in summer – was 36.8±2.3mmd−1 and 23.0±1.9mmd−1 for hydroponic wetlands planted with cattail and common reed, respectively), although P. australis water use efficiency was lower. Positive relationships were found between ET and EH for planted wetlands. Cattail presented a stronger linear regression than common reed, demonstrating that ET and consequently redox conditions are plant species-dependent.

The hydrological characterisation and water budget of a South African rehabilitated headwater wetland system

This paper presents a synopsis of the findings of a valley bottom wetland monitoring study in which dominant hydrological processes maintaining the system are quantitatively defined. The Craigieburn-Manalana is a wetland subjected to technical rehabilitation, at the headwaters of the Sand River in the lowveld savanna region of South Africa. Findings include the identification of a rapid water delivery mechanism from the surrounding hillslopes to the wetland following a threshold-exceeding precipitation event, when hillslope-toe soil matric potential is close to 0, leading to a raising of the wetland water table by >0.7 m within 3 h. A summary of quantified fluxes and associated water budget of the wetland and its contributing catchment is developed. It is revealed that this wetland does not necessarily conform to the typical assumptions that wetlands augment low flows. Surface layer scintillometry shows actual wetland evapotranspiration to dominate the water budget during the dry season (2.3–3.5 mm/d) compared to its contributing catchment (0.9–2.2 mm/d), whilst stream discharge had ceased. Hydrograph separation, based on stable isotopes (18O), revealed that the wetland does not attenuate peak flows during the summer rains when the wetlands soil moisture deficit is close to 0, since more than 66% of stream discharge comprised event water. These results are discussed within the context of current hydrological understanding of southern African headwater wetlands, such as dambos. Keywords: hillslope processes, hydro-geomorphology, water budget, dambos, rehabilitation, wetlands

Nonlinear controls on evapotranspiration in arctic coastal wetlands

Abstract. Projected increases in air temperature and precipitation due to climate change in Arctic wetlands could dramatically affect ecosystem function. As a consequence, it is important to define controls on evapotranspiration, the major pathway of water loss from these systems. We quantified the multi-year controls on midday Arctic coastal wetland evapotranspiration, measured with the eddy covariance method at two vegetated, drained thaw lake basins near Barrow, Alaska. Variations in near-surface soil moisture and atmospheric vapor pressure deficits were found to have nonlinear effects on midday evapotranspiration rates. Vapor pressure deficits (VPD) near 0.3 kPa appeared to be an important hydrological threshold, allowing latent heat flux to persistently exceed sensible heat flux. Dry (compared to wet) soils increased bulk surface resistance (water-limited). Wet soils favored ground heat flux and therefore limited the energy available to sensible and latent heat flux (energy-limited). Thus, midday evapotranspiration was suppressed from both dry and wet soils but through different mechanisms. We also found that wet soils (ponding excluded) combined with large VPD, resulted in an increased bulk surface resistance and therefore suppressing evapotranspiration below its potential rate (Priestley-Taylor α < 1.26). This was likely caused by the limited ability of mosses to transfer moisture during large atmospheric demands. Ultimately, in addition to net radiation, the various controlling factors on midday evapotranspiration (i.e., near-surface soil moisture, atmospheric vapor pressure, and the limited ability of saturated mosses to transfer water during high VPD) resulted in an average evapotranspiration rate of up to 75% of the potential evapotranspiration rate. These multiple limitations on midday evapotranspiration rates have the potential to moderate interannual variation of total evapotranspiration and reduce excessive water loss in a warmer climate. Combined with the prevailing maritime winds and projected increases in precipitation, these curbing mechanisms will likely prevent extensive future soil drying and hence maintain the presence of coastal wetlands.

Evapotranspiration from subsurface horizontal flow wetlands planted with Phragmites australis in sub-tropical Australia

The balance between evapotranspiration ( ET) loss and rainfall ingress in treatment wetlands (TWs) can affect their suitability for certain applications. The aim of this paper was to investigate the water balance and seasonal dynamics in ET of subsurface horizontal flow (HF) TWs in a sub-tropical climate. Monthly water balances were compiled for four pilot-scale HF TWs receiving horticultural runoff over a two year period (Sep. 1999–Aug. 2001) on the sub-tropical east-coast of Australia. The mean annual wetland ET rate increased from 7.0 mm/day in the first year to 10.6 mm/day in the second, in response to the development of the reed ( Phragmites australis) population. Consequently, the annual crop coefficients (ratio of wetland ET to pan evaporation) increased from 1.9 in the first year to 2.6 in the second. The mean monthly ET rates were generally greater and more variable than the Class-A pan evaporation rates, indicating that transpiration is an important contributor to ET in HF TWs. Evapotranspiration rates were generally highest in the summer and autumn months, and corresponded with the times of peak standing biomass of P. australis. It is likely that ET from the relatively small 1 m wide by 4 m long HF TWs was enhanced by advection through so-called “clothesline” and “oasis” effects, which contributed to the high crop coefficients. For the second year, when the reed population was well established, the annual net loss to the atmosphere (taking into account rainfall inputs) accounted for 6.1–9.6 % of the influent hydraulic load, which is considered negligible. However, the net loss is likely to be higher in arid regions with lower rainfall. The Water Use Efficiency ( WUE) of the wetlands in the second year of operation was 1.3 g of above-ground biomass produced per kilogram of water consumed, which is low compared to agricultural crops. It is proposed that system level WUE provides a useful metric for selecting wetland plant species and TW design alternatives to use in arid regions where excessive water loss from constructed wetlands can be problematic. Further research is needed to accrue long-term HF TW water balance data especially in arid climatic zones.

Partitioning of vertical water loss in reed swamp wetlands: theory, research and application

This paper proposes an on-site monitoring system for evaporation (Ew) and evapotranspiration (ET) in wetlands called the Compensation Evapotranspiration Observation System. Considering the characteristics of wetlands environment and emergent plants, the vertical flux of water loss in wetlands can be separated using the system based on compensation principle. A comparison research on water loss through ET in reed (Phragmites australis) stands between Zhalong wetland and Baiyangdian wetland was carried out in 2004-2009. In Baiyangdian wetland, the ET rates from three reed stands with different leaf area indexes were measured. The results show that annual ET in Zhalong wetland was 739–886 mm during study period. Comparatively, annual ET of the three reed stands in Baiyangdian wetland was 970–1117 mm, 1261–1452 mm and 1759–2035 mm, respectively. The ET rates in Baiyangdian were stronger than in Zhalong because of the effects of geography and climate characteristics. Moreover, seasonal dynamic changes and influence factors of transpiration-acting factor from the two wetlands were also analyzed. It is indicated that the transpiration-acting factor can be affected by geographic location, aboveground biomass, plant growth and climate conditions.

Wetlands as energy-dissipating systems

Since wetlands are ecosystems that have an ample supply of water, they play an important role in the energy budgets of their respective landscapes due to their capacity to shift energy fluxes in favor of latent heat. Rates of evapotranspiration in wetlands are commonly as high as 6-15 mm day⁻¹, testifying to the large amount of energy that is dissipated through this process. Emergent or semi-emergent wetland macrophytes substantially influence the solar energy distribution due to their high capacity for transpiration. Wetland ecosystems in eutrophic habitats show a high primary production of biomass because of the highly efficient use of solar energy in photosynthesis. In wetlands associated with the slow decomposition of dead organic matter, such as oligotrophic marshes or fens and bogs, the accumulation of biomass is also high, in spite of the rather low primary production of biomass. Most of the energy exchange in water-saturated wetlands is, however, linked with heat balance, whereby the largest proportion of the incoming energy is dissipated during the process of evapotranspiration. An example is shown of energy fluxes during the course of a day in the wetland ecosystem of Mokré Louky (Wet Meadows) near Třeboň. The negative consequences of the loss of wetlands for the local and regional climate are discussed.

Estimating the regional evapotranspiration in Zhalong wetland with the Two-Source Energy Balance (TSEB) model and Landsat7/ETM+ images

This paper aims to integrate a numerical model, vegetation index, and regional evapotranspiration estimation to assess the water cycle in Zhalong wetland, China with the aid of remote sensing technologies. An interdisciplinary analysis was performed to study the eco-hydrological characteristics of the wetland ecosystem. In particular, a new solution for solving the Two-Source Energy Balance (TSEB) was developed and applied. This new solution method is based on a definition of the aerodynamic temperature in a two-layer ground surface structure. Following this new solution for TSEB, the outputs of the Surface Energy Balance Algorithm (SEBAL) may be used as the inputs for TSEB to avoid the complex calculations of the bulk boundary layer and soil surface resistance. The new solution method makes the implementation of TSEB much easier when bi-directional thermal infrared remote sensing images are not available. Daily evapotranspiration and vegetation index of the wetland were calculated from six scenes of Landsat7/ETM+ remote sensing images acquired during 2001 and 2002 in our case study. Spatial distribution of daily evapotranspiration was also provided to help realize the wetland condition. The seasonal changes of vegetation index and evapotranspiration for some typical wetland ground surfaces were analyzed and presented as well to explore the multitemporal variations of plant species holistically. Ultimately, such integrative analysis aids in understanding the general biological patterns associated with these wetland vegetation cover, while specific situations of the wetland ecosystem can be recognized.

Surface vegetation controls on evapotranspiration from a sub‐humid Western Boreal Plain wetland

AbstractWetlands in the Western Boreal Plain (WBP) of North Central Alberta exist within a moisture‐deficit regime where evapotranspiration (ET) is the dominant hydrologic flux. As such these systems are extremely susceptible to the slightest climatic variability that may upset the balance between precipitation (P) and ET. Wetland ET is predominantly controlled by vegetation composition but may also vary due to moisture regimes and microclimatic factors. To address this variability in moisture regimes, ET was examined in a typical moraine‐wetland‐pond system of the WBP during the 2005 and 2006 snow‐free seasons. Closed dynamic chamber measurements were used to gather data on plant community‐scale actual evapotranspiration (ET) in an undisturbed natural bog with varying degrees of canopy cover surrounding a shallow groundwater‐fed pond. For the purposes of scaling plant community ET contributions to those of the wetland, potential ET (PETEQ) was measured using a Priestley–Taylor energy balance approach at three separate wetland sites with varying aspects surrounding the central pond, along with actual evapotranspiration using a roving eddy covariance (EC) tower. Growing season peak ET rates ranged from 0·2 mm/h to 0·6 mm/h depending on the location, vegetation composition and time period. Sphagnum contributions were the greatest early in the growing season, reaching peaks of 0·6 mm/h, while lichen sites exhibited the greatest late season rates at 0·4 mm/h. Thus, Sphagnum and other nonvascular wetland plant species control ET differently throughout the growing season and as such should be considered an integral part of the moisture and water balances within wetland environments at the sub‐landscape unit scale. Copyright © 2010 John Wiley & Sons, Ltd.

Evapotranspiration and water use efficiency in a Chesapeake Bay wetland under carbon dioxide enrichment

AbstractWetlands evapotranspire more water than other ecosystems, including agricultural, forest and grassland ecosystems. However, the effects of elevated atmospheric carbon dioxide (CO2) concentration (Ca) on wetland evapotranspiration (ET) are largely unknown. Here, we present data on 12 years of measurements of ET, net ecosystem CO2 exchange (NEE), and ecosystem water use efficiency (EWUE, i.e. NEE/ET) at 13:00–15:00 hours in July and August for a Scirpus olneyi (C3 sedge) community and a Spartina patens (C4 grass) community exposed to ambient and elevated (ambient+340 μmol mol−1) Ca in a Chesapeake Bay wetland. Although a decrease in stomatal conductance at elevated Ca in the S. olneyi community was counteracted by an increase in leaf area index (LAI) to some extend, ET was still reduced by 19% on average over 12 years. In the S. patens community, LAI was not affected by elevated Ca and the reduction of ET was 34%, larger than in the S. olneyi community. For both communities, the relative reduction in ET by elevated Ca was directly proportional to precipitation due to a larger reduction in stomatal conductance in the control plants as precipitation decreased. NEE was stimulated about 36% at elevated Ca in the S. olneyi community but was not significantly affected by elevated Ca in S. patens community. A negative correlation between salinity and precipitation observed in the field indicated that precipitation affected ET through altered salinity and interacted with growth Ca. This proposed mechanism was supported by a greenhouse study that showed a greater Ca effect on ET in controlled low salinity conditions compared with high salinity. In spite of the differences between the two communities in their responses to elevated Ca, EWUE was increased about 83% by elevated Ca in both the S. olneyi and S. patens communities. These findings suggest that rising Ca could have significant impacts on the hydrologic cycles of coastal wetlands.

Contrasting carbon dioxide fluxes between a drying shrub wetland in Northern Wisconsin, USA, and nearby forests

Abstract. Wetland biogeochemistry is strongly influenced by water and temperature dynamics, and these interactions are currently poorly represented in ecosystem and climate models. A decline in water table of approximately 30 cm was observed at a wetland in Northern Wisconsin, USA over a period from 2001–2007, which was highly correlated with an increase in daily soil temperature variability. Eddy covariance measurements of carbon dioxide exchange were compared with measured CO2 fluxes at two nearby forests in order to distinguish wetland effects from regional trends. As wetland water table declined, both ecosystem respiration and ecosystem production increased by over 20% at the wetland, while forest CO2 fluxes had no significant trends. Net ecosystem exchange of carbon dioxide at the wetland was not correlated with water table, but wetland evapotranspiration decreased substantially as the water table declined. These results suggest that changes in hydrology may not have a large impact on shrub wetland carbon balance over inter-annual time scales due to opposing responses in both ecosystem respiration and productivity.

Connecting Atmosphere and Wetland: Energy and Water Vapour Exchange

AbstractWetlands are ubiquitous over the globe, comprise a vast array of ecosystem types and are of great ecological and social importance. Their functioning is intimately tied to the atmosphere by the energy and mass exchanges that take place across the wetland–atmosphere boundary. This article examines recent research into these exchanges, with an emphasis on the water vapour exchange. Although broad classes of wetland type, such as fen, bog and marsh, can be defined using ecological or hydrologic metrics, distinct difference in energy exchanges between the classes cannot be found. This arises because there are many factors that control the energy exchanges and interplay of these factors is unique to every wetland ecosystem. Wetlands are more similar in their radiation balances than in the partitioning of this energy into conductive and turbulent heat fluxes. This is especially true of evapotranspiration (ET) rates, which vary considerably among and within wetland classes. A global survey of wetland ET studies shows that location has little to do with ET rates and that variation in rates is largely determined by local climate and wetland characteristics. Recent modelling studies suggest that although wetlands occupy a small portion of the global land surface, their water and energy exchanges may be important in regional and global climates. Although the number of studies of wetland–atmosphere interactions has increased in recent years more research is needed. Five key areas of study are identified: (i) the importance of moss covers, (ii) lack of study in tropical systems, (iii) inclusion of wetlands in global climate models, (iv) importance of microforms in wetlands and their scaling to the whole ecosystem, and (v) the paucity of annual ET measurements.

Ground water discharge by evapotranspiration in wetlands of an arid intermountain basin

Summary To improve basin-scale modeling of ground water discharge by evapotranspiration (ET) in relation to water table depth, daily ET was measured using the Bowen ratio energy balance method during 1999–2005 in five herbaceous plant dominated wetlands in an arid intermountain basin in Colorado, USA. Three wetlands were wet meadows supplied primarily by regional ground water flow and two were playas supplied primarily by local stream flow. In wet meadows, mean daily water table depth (WTD) ranged from 0.00 m (ground surface) to 1.2 m, with low inter-annual variability. In wet meadows, annual actual ET (ETa) was 751–994 mm, and ground water discharge from the shallow aquifer (ETg) was 75–88% of ETa. In playas, mean daily WTD ranged from � 0.65 to 1.89 m, with high inter-annual variability. In playas, annual ETa was 352–892 mm, and ETg was 0–77% of ETa. The relationship of annual ETg to WTD was compared to existing ETg– WTD models. For wet meadows, ETg decreased exponentially as WTD increased from 0.13 to 0.95 m (r 2 = 0.83, CV = 5%, p < 0.001). In comparison with our findings, existing models under- and over-estimate ETg by � 30% to 47% at WTD of 0.13 m, and they under-estimate ETg by � 12% to � 42% at WTD of 0.95 m. This study found that as the water table declined from near the soil surface to 0.95 m, ETg decreased only � 26% versus 39– 55% estimated by existing models. The magnitude of ETg decrease was 220 mm, whereas existing models predicted decreases up to 700 mm (218% greater). In playas, there was no clear ETg–WTD relationship. Instead, ETg was strongly dependent on the surface water supply. When sufficient surface water inputs occurred to meet ET demand, ETg was � 0 mm/yr and independent of WTD. When inputs did not meet ET demand, ETg was positive though highly variable at WTD up to 1.68 m.

Factors that control Typha marsh evapotranspiration

There is continuing debate about the controls on wetland evapotranspiration ( E t) and whether marshes are profligate water users. We used eddy covariance to measure the CO 2 exchange and E t by a California Tule marsh in 2003. The marsh was dominated by Typha and Scirpus, and there was a large amount of standing litter that acted as a mulch. Canopy development was broadly related to air temperature, with rapid growth in May and senescence in October. E t was a few tenths of a mm d −1 in winter, and 3–4 mm d −1 in summer. The midsummer Bowen ratio was ∼1, and the annual E t was 49 cm. The peak rate of E t was lower than has been reported for marshes based on lysimeter studies, somewhat lower than has been reported for marshes based on micrometeorological studies, and equivalent to, or somewhat lower than, has been reported for upland grassland. The midsummer water use efficiency was 0.0025 mol CO 2 mol −1 H 2O, and the δ 13C of foliage was −27.1‰, which are both typical for productive C 3 ecosystems. Transpiration accounted for 80% of total E t. Evaporation from water standing beneath the canopy and mulch layer was only a minor component of the marsh's hydrological budget. The low rate of evaporation from standing water was a result of cool water temperatures, which remained within a few degrees of the nocturnal minimum on most days. We believe the mulch layer acted in a way analogous to an electrical diode that allowed the upward loss of heat from the water to the atmosphere at night, and shut off the flux of heat from the atmosphere to the water during daytime, resulting in cool subcanopy water and low rates of evaporation. Our observations are inconsistent with the hypothesis that Tule marshes are inefficient water users, or that their rates of transpiration and CO 2 uptake are unusual compared to upland ecosystems.

Effects of climatic change on evapotranspiration in Zhalong Wetland, Northeast China

Evapotranspiration (ET) process of plants is controlled by several factors. Besides the physiological factors of plants, height, density, LAI (leaf area index), etc., the change of meteorological factors, such as radiation, temperature, wind and precipitation, can influence ET process evidently, thus remodeling the spatial and temporal distribution of ET. In order to illuminate the effects of meteorological factors on wetland ET, the ET of Zhalong Wetland was calculated from 1961 to 2000, the statistical relationships (models) between ET and maximum temperature (T max), minimum temperature (T min), precipitation (P) and wind speed at 2m height (U 2) were established, and the sensitivity analysis of the variables in the model was performed. The results show that T max and T min are two dominating factors that influence ET markedly, and the difference of rising rate between T max and T min determines the change trend of ET. With the climatic scenarios of four General Circulation Models (GCMs), the ET from 2001 to 2060 was predicted by the statistical model. Compared to the period of 1961–2000, the water consumption by ET will increase greatly in the future. According to the scenarios, the rise of T max (about 1.5°C to 3.3°C) and T min (about 1.7°C to 3.5°C) will cause an additional water consumption of 14.0%–17.8% for reed swamp. The ecological water demand in Zhalong Wetland will become more severe.

Alternate corrections for estimating actual wetland evapotranspiration from potential evapotranspiration

Corrections can be used to estimate actual wetland evapotranspiration (AET) from potential evapotranspiration (PET) as a means to define the hydrology of wetland areas. Many alternate parameterizations for correction coefficients for three PET equations are presented, covering a wide range of possible data-availability scenarios. At nine sites in the wetland Everglades of south Florida, USA, the relatively complex PET Penman equation was corrected to daily total AET with smaller standard errors than the PET simple and Priestley-Taylor equations. The simpler equations, however, required less data (and thus less funding for instrumentation), with the possibility of being corrected to AET with slightly larger, comparable, or even smaller standard errors. Air temperature generally corrected PET simple most effectively to wetland AET, while wetland stage and humidity generally corrected PET Priestley-Taylor and Penman most effectively to wetland AET. Stage was identified for PET Priestley-Taylor and Penman as the data type with the most correction ability at sites that are dry part of each year or dry part of some years. Finally, although surface water generally was readily available at each monitoring site, AET was not occurring at potential rates, as conceptually expected under well-watered conditions. Apparently, factors other than water availability, such as atmospheric and stomata resistances to vapor transport, also were limiting the PET rate.

Simulating riparian evapotranspiration: a new methodology and application for groundwater models

This paper describes the development and application of a new methodology to simulate riparian and wetland evapotranspiration (ET) in groundwater models. Traditional approaches for modeling ET are based on quasi-linear relationship between ET flux rate and hydraulic head (groundwater elevation). The approach presented here uses multiple non-linear, segmented flux curves that reflect the ecophysiology of the plant species in these systems. Five plant functional groups (PFGs) based on water tolerance ranges and rooting depths are used to elucidate the interactive processes of plant transpiration with groundwater conditions. ET flux rate curves set the extinction and saturation extinction depths and define the group's ET flux rate as a function of water table depth relative to the ground surface. The calculated ET loss from a riparian or wetland system is dependent on the contributing area of each plant functional subgroup present and water table conditions. The new methodology requires a fractional coverage for each of the PFGs present within the groundwater model cell and allows for more accurate assignment of land surface elevations. Model results for a case-study show significant differences in predicted ET and subsequent depth to groundwater between the new and the traditional approaches. The development of physiologically based transpiration curves combined with the traditional linear curve for bare soil/open water results in more accurate determinations of riparian ET and improved basin scale water budgets. The use of PFGs in combination with the new RIP-ET package provides an explicit link between groundwater and riparian/wetland habitat conditions and offers an opportunity to better manage and restore riparian and wetland systems.

Evapotranspiration from a riparian fen wetland

Evapotranspiration rates were measured in a riparian fen wetland dominated by vascular vegetation and surrounded by open agricultural areas and forests. The wetland is situated on a floodplain in central Denmark. Measurements were taken throughout the growing season (April–September) of 1999. Evapotranspiration rates were higher than those published for most other wetland types, with an average of 3.6 mm d−1 during the growing season and a peak rate of 5.6 mm d−1. Daily average evapotranspiration was 110% of Penman's potential open water evaporation. Evapotranspiration was the dominant sink in the energy balance of the wetland studied. During the day, evapotranspiration accounted for 82% of the available radiant energy, Rn. Due to the presence of deposited fine-grained sediments, soil-water availability was kept high at all times which resulted in moderate canopy resistances, rc (overall mean =32 s m−1). Evapotranspiration was controlled by a combination of driving forces: Rn, saturation vapour pressure deficit, D, and rc. It is hypothesized that the results presented in this study are conditioned by the proximity of the wetland to drier upland areas. During periods with high evaporative demand and low precipitation, warm, dry air is formed over the upland areas and wetland evapotranspiration rates are enhanced by local advection. Indicative evidence for the hypothesis is presented. Although the absolute magnitude of the results reported is only directly relevant to similar sites in Denmark, the processes and controls described are considered to be representative of riparian wetlands subjected to frequent flooding and/or with a high groundwater table, with vascular vegetation, and which are narrow corridors in open agricultural landscapes.