Browsing by Subject "Evapotranspiration"

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    Comparison of Evapotranspiration Estimation Methods and Implications for Water Balance Model Parameterization in the Midwestern United States
    (2019-12) Talbot, Michael
    Evapotranspiration is the second most dominant component of the global water cycle behind precipitation, yet it remains one of the most difficult to measure and model. The numerous methods that have been developed for estimating evapotranspiration (ET) rates using climatological data vary in both complexity and spatiotemporal robustness. While the Penman-Monteith method has continually been shown to compare better with observed ET rates across more geographies and timescales than any other method, its high data requirements remain a barrier to use in many areas, and it is often desirable or necessary to make use of an alternative method. Daily reference ET estimates from the Penman-Monteith method were compared to ET estimates from seven alternative methods, which were generated using 14 years of observed weather records at five locations across the Midwestern United States. Then, a one-dimensional water balance model, DRAINMOD, was run at 362 locations across the Midwest using 50 years of synthetic climate data and three distinct sets ET inputs: 1) reference ET from the Penman-Monteith method, 2) potential ET generated from the Penman-Monteith reference ET and location-specific crop coefficient curves, and 3) potential ET from the Thornthwaite method. Results suggest that the best alternative method to Penman-Monteith varies by location, application, and timescale of interest, and that the misapplication of ET estimates for water balance model parameterization could have a dramatic impact on the accuracy of model predictions.
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    Constraining Regional Evapotranspiration in the Upper Midwestern United States Using In Situ Observations and Numerical Modeling
    (2023-08) Xiao, Ke
    Evapotranspiration (ET) is a critical component in the global water cycle and water resource management. The Upper Midwestern United States (US), a major agricultural production region with large areas of lake and cropland, is facing challenges related to extreme variations in precipitation, increasing irrigation water usage, and large fluctuations in water budgets. Lake evaporation and cropland ET represent two significant components of the regional water budget. However, regional ET estimates from these two sources contain large uncertainties due to their complex interactions with atmospheric conditions (e.g. precipitation) and land surface processes (e.g. ice/plant phenology, atmospheric demand), affected by climate change and anthropogenic activities. This dissertation combines in situ observations and modeling to better constrain the regional ET, focusing on lake evaporation, local water recycling, and cropland ET in the Upper Midwest US.Evaporation from a temperate closed-basin lake, White Bear Lake (WBL), was estimated using the eddy covariance method and an optimized lake model CLM4-LISSS. The annual evaporation totals from 2014 to 2016 were 559 ± 22 mm, 779 ± 81 mm, and 766 ± 11 mm, respectively. The combined effects of smaller average daily evaporation and a shorter ice-free season caused lower evaporation in 2014. Retrospective analyses indicated that WBL evaporation increased by 3.8 mm/year during 1979–2016, which was driven by increased wind speed and lake-surface vapor pressure gradient. Lake evaporation is expected to increase by 1.4 mm/year from 2017 to 2100 under the business-as-usual greenhouse gas emission scenario, largely driven by extended ice-free periods. These results imply that the water level of WBL is closely coupled to evaporation and consequently impacted by the large-scale synoptic and climatic conditions. The contribution of ET to regional precipitation, known as “local water recycling”, is a key process in the water cycle. An idealized two-layer equilibrium planetary boundary layer model was coupled with a stable isotope module that included HDO and H218O in water to constrain the local water recycling ratio (LRR) by isotope observations. The regional value of the summer LRR was estimated to be 0.29 ± 0.12. The summer LRR values for the years 2006–2010 varied between 0.17 and 0.36. The smallest value of LRR was in 2008 which corresponded to a drought year. Cropland has likely changed the regional LRR by −7.6 to 19.5% under different pre-agriculture land cover scenarios. The model also implies that local water recycling is expected to be weakened under drought conditions, but it will be enhanced if irrigation is applied more intensely. In humid continental climates, forecasting cropland ET is challenging due to the variable precipitation and plant phenology. An ET forecast system (ETool) was built upon the Weather Research and Forecasting (WRF) model with the Noah land surface scheme to forecast the weekly ET at 3-km resolution in Minnesota, US. The near real-time leaf area index (LAI) from the Moderate Resolution Imaging Spectroradiometer (MODIS) product was used in ETool to improve the representation of plant phenology. At a cropland site, the LAI improvement led to a 17.7% reduction in the weekly ET forecast bias, with an R2 value of 0.82 and a root-mean-square error of 0.64 mm/day. Using the predicted difference between precipitation and ET, ETool can inform irrigation scheduling to balance the tradeoff between safeguarding yields and conserving water usage. Collectively, this dissertation also revealed the feedback processes between ET and climate under the influence of anthropogenic activities in the Upper Midwest US. As the climate continues to warm, the regional lake evaporation is expected to increase with lengthening ice-free periods. The regional cropland ET and local water recycling are expected to be weakened due to an increased likelihood of drought events. However, irrigation is anticipated to increase in response to the more frequent drought events, which will conversely enhance ET and local water recycling. Furthermore, more intense groundwater usage and greater fluctuations in lake water levels within the region are expected with increased use of irrigation.
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    The impact of trees on temporal variability in urban carbon and water budgets.
    (2010-07) Peters, Emily Beth
    Urbanization is responsible for some of the fastest rates of land-use change around the world, with important consequences for local, regional, and global climate. Vegetation can represent a significant proportion of many urban and suburban landscapes and modifies climate by altering local exchanges of heat, water vapor, and CO2. To assess the contribution of plant functional types to urban ecosystem processes of water loss and carbon gain in a suburban neighborhood of Minneapolis-Saint Paul, Minnesota, USA, I investigated 1) the microclimate effects of different forest types over time, 2) the relative importance of environmental and biological controls on urban tree transpiration and canopy photosynthesis, and 3) the relative importance of trees and turfgrass on the spatial and seasonal variation in suburban evapotranspiration. Regardless of plant functional type, I found that seasonal patterns of soil and surface temperature were controlled by differences in stand-level leaf area index, and that sites with high leaf area index had soil and surface temperatures 7°C and 6°C lower, respectively, than sites with low leaf area index. Plant functional type differences in canopy structure and growing season length largely explained why evergreen needleleaf trees had significantly higher annual transpiration (307 kg H2O m-2 yr-1) and canopy photosynthesis (1.02 kg C m-2 yr-1) rates per unit canopy area than deciduous broadleaf trees (153 kg H2O m-2 yr-1 and 0.38 kg C m-2 yr-1, respectively), offering an approach to scale up the tree component of urban water and carbon budgets. Turfgrass represented the largest contribution to annual evapotranspiration in recreational and residential land-use types (87% and 64%, respectively), due to a higher fractional cover and greater daily water use than trees. Component-based estimates of suburban evapotranspiration underestimated measured water vapor fluxes by 3%, providing a useful approach to predict the seasonal patterns of evapotranspiration in cities. These finding have implications for the management of urban ecosystem services related to climate, carbon sequestration, and hydrology, and for predicting the impacts of climate change on urban ecosystems.
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    River Nitrogen Loads and Landscape Evapotranspiration as Influenced by Climate and Land Cover Changes in the Midwestern United States
    (2018-06) Baeumler, Nathaniel
    The Northern Gulf of Mexico is threatened by the yearly development of a hypoxic zone (dissolved oxygen <2 mg/L); also called the “dead zone”. The areal extent of the dead zone has been linked to increased outflow of freshwater and nutrients, especially soluble nitrogen from the Mississippi River Basin to the Gulf. Much of the expansion of the hypoxic zone has been blamed on land cover changes in the last two centuries; especially the installation of subsurface drainage and adoption of row crops over native vegetation. The research presented in this thesis set out to understand (1) how climate and land cover changes have impacted flow and N loads in rivers of the Midwestern United States; the main source of soluble N to the Mississippi River, and (2) the extent of changes in landscape evapotranspiration (ET) from adoption of corn and soybeans relative to native prairies. In the first chapter of the thesis, we evaluated the role of precipitation and land cover changes on streamflow, baseflow, flow weighted N concentrations (FWNC), and N loads in various rivers of the Midwestern United States. This quantification was done using the stepwise regression analysis both at the annual and monthly scales. For annual analysis, the predictor variables were the Ln(annual streamflow), Ln(annual baseflow), annual FWNC and Ln(annual N loads) and the explanatory variables were the annual precipitation in the current year (P1), and previous years (P2, P3), and area under soybean production (Soy). For monthly analysis, the predictor variables were the Ln(monthly streamflow), Ln(monthly baseflow), monthly FWNC, and Ln(monthly N Loads) and the explanatory variables were the area under soybean production, precipitation in the previous year, and precipitation of the current, and all previous months up to that point in the season. The annual analysis showed that streamflows and baseflows were often controlled not only by the current year precipitation, but also by the previous year precipitation. The previous year precipitation effects were in terms of increased or decreased soil wetness (fillable porosity). In some instances, previous year precipitation effects on streamflow or baseflow lingered for up to 2-3 years. The area under soybean production was generally not a significant explanatory variable in the annual analysis likely due to its small variations during the study periods. In the monthly analysis, precipitation in both the current month and previous months along with the previous year precipitation were important in controlling Ln(monthly streamflow), Ln(monthly baseflow), and Ln(monthly N loads). Area under soybean production was significant in some months but had much lower statistical power than the precipitation variables. Besides the direct effects of precipitation (increased or decreased streamflow and baseflow) on N loads, there were also some indirect effects. These included substantial N left in the soil profile during dry years, which on subsequent normal precipitation years lead to a large spike in river N loads. Conversely, there was less N left in soils in wet years, which lessened the N losses in subsequent years. Analysis of the combined data from seven rivers in the Midwestern United States showed similar relationship between Ln(annual streamflow) with current year and previous year precipitations and the area under soybean production. However, the area under soybean production only explained 1% of the variability in Ln(annual streamflow). Compared to Ln(annual streamflow), the combined plots of Ln(annual baseflow) versus annual precipitation showed some differences among watersheds. These differences appeared to be related to differences in landscape steepness, extent of tile drainage, soil water holding capacity, etc. among various watersheds. Because of the differences in annual baseflow among various rivers, there were also some differences in Ln(annual N yield) versus annual precipitation relationships among various rivers. These differences appeared to be related to watershed characteristics such as steepness and soil available N both from fertilizer input and mineralization from soil organic matter. This analysis showed that less water percolation through the soil will help stretch the N yield versus precipitation curves such that there are much lower N losses even at higher annual precipitations. Two possible ways to reduce water percolation is through increased overland flow (via surface inlets) and through introduction of high ET crops. However, the presence of surface inlets in the landscape increases the potential of more sediment and phosphorus losses to rivers. High ET crops include Miscanthus and switch grass. However, there is conflicting information in the literature on ET from these crops relative to corn and soybeans. Furthermore, there are challenges in establishing the Miscanthus in cold climate due to winterkill. Another way to reduce N losses to rivers may be through less N input on the landscape. However, applied inorganic N fertilizer was not a significant variable in explaining variability in river N loads. This may be partially due to limited variation in applied N fertilizer rates as well as limited number of years over which both the N loss and N fertilizer data were available. In the second chapter, we evaluated the evapotranspiration from irrigated and non-irrigated corn, non-irrigated soybeans, and native prairies at various stage of management in Western Minnesota. ET evaluation was done using the “Mapping Evapotranspiration at High Resolution using Internalized Calibration” (METRIC) model for the 2015 growing season. The inputs included satellite imagery and weather data. The results from this study showed that estimated ET from 8 June through 30 September 2015 followed the trend: wetland (671 mm)> non-irrigated corn (627 mm)>irrigated corn (601 mm)>non-irrigated soybeans (534 mm)>previously burned prairie (532 mm)>recently burned prairie (397 mm). Comparatively for the period 1 May to 30 September 2015, the estimated ET of wetland, previously burned and recently burned prairie grasses were 810 mm, 633 mm and 485 mm, respectively. These modeling estimates were similar to the literature values and showed that ET of native prairie grasses is nearly similar or marginally higher than that of corn and soybeans. Considering that soybeans also replaced low transpiring small grains such as oats and wheat suggests that large changeover of vegetative cover replacing native prairie and small grains with soybeans starting in 1940s in the Upper Midwestern United States likely had a very small to minimal impact on landscape evapotranspiration.
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    Understory Transpiration Rates Following Stand Density Reduction in a Coast Redwood Forest
    (2020-08) Hammerschmidt, Shelby
    In forests where the overstory canopy has been disturbed, evapotranspiration (ET) by the understory may be the main flux of water back to the atmosphere. The ability to take field measurements of water use by understory plants, therefore, is vital for a complete ecosystem water budget. However, little research has been apportioned to directly measuring understory water use, and the technology to do so is thus limited. Portable ET chambers have been used to measure ET rates in agricultural fields, grasslands, and deserts, but not in a forest understory. Thus, a portable rapid chamber which can be easily deployed and collect quick ET measurements of single plants was developed for measuring understory plant water use in logged watersheds in coastal California. Mean understory ET rate was highest in the watershed with the lowest residual basal area ( = 87  56 mm/day) and lowest in the control watershed ( = 31  19 mm/day). Multiple regression modeling indicates that the difference in ET rate between watersheds is caused by freed soil water as a result of overstory tree removal. These results imply that understory water use is likely significant in harvested watersheds, and should be quantified at the landscape scale.
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    Water yield in the southern Appalachian Mountains.
    (2011-05) Kove, Katherine Marie
    With over 55% forest cover, the southern Appalachians (SA) are a main water resource for the surrounding areas. These water resources are at risk due to changing climate and precipitation regimes as well as changes in forest cover. Understanding the implications of these risks will help to develop management strategies for an increasingly valuable resource. Evapotranspiration (ET), the combination of plant transpiration and surface evaporation, can vary across space and time, and is a significant component of the hydrological cycle in densely forested regions. Quantifying ET is critical to understanding the available water resource, especially in the SA. In the SA, ET averages 50% of annual precipitation in forested watersheds and can climb to 85%. However, ET is among the most difficult and complex component of the water cycle to measure and model. This dissertation addresses these complexities by investigating the ability of sap flow models to estimate ET and examining the impact of potential temperature and compositional shifts on water yield. We also examined sap flow input variables to determine the best methods for the SA including the spatial estimation of climatological variables, phenological dates, and leaf area index (LAI) estimates all of which would particularly enhance the development of our hydrological models.