In this first worldwide synthesis of in situ and satellite‐derived lake data, we find that lake summer surface water temperatures rose rapidly (global mean = 0.34°C decade−1) between 1985 and 2009. Our analyses show that surface water warming rates are dependent on combinations of climate and local characteristics, rather than just lake location, leading to the counterintuitive result that regional consistency in lake warming is the exception, rather than the rule. The most rapidly warming lakes are widely geographically distributed, and their warming is associated with interactions among different climatic factors—from seasonally ice‐covered lakes in areas where temperature and solar radiation are increasing while cloud cover is diminishing (0.72°C decade−1) to ice‐free lakes experiencing increases in air temperature and solar radiation (0.53°C decade−1). The pervasive and rapid warming observed here signals the urgent need to incorporate climate impacts into vulnerability assessments and adaptation efforts for lakes.

Water table depth (WTD), soil texture, and growing season weather conditions all play critical roles in determining agricultural yield; however, the interactions among these three variables have never been explored in a systematic way. Using a combination of field observations and biophysical modeling, we answer two questions: (1) under what conditions can a shallow water table provide a groundwater yield subsidy and/or penalty to corn production?; and (2) how do soil texture and growing season weather conditions influence the relationship between WTD and corn yield?. Subfield-scale yield patterns during a dry (2012) and wet (2013) growing season are used to identify sensitivity to weather. Areas of the field that are negatively impacted by wet growing seasons have the shallowest observed WTD (<1 m), while areas with consistently strong yield have intermediate WTD (1-3 m). Parts of the field that perform consistently poorly are characterized by deep WTD (>3 m) and coarse soil textures. Modeling results find that beneficial impacts of shallow groundwater are more common than negative impacts under the conditions studied, and that the optimum WTD is shallower in coarser soils. While groundwater yield subsidies have a higher frequency and magnitude in coarse-grained soils, the optimum WTD responds to growing season weather at a relatively constant rate across soil types. We conclude that soil texture defines a baseline upon which WTD and weather interact to determine overall yield. Our work has implications for water resource management, climate/land use change impacts on agricultural production, and precision agriculture.

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[1] Evapotranspiration (ET) is a significant component of the water and energy balance in wetlands and riparian zones, yet it is also one of the most challenging components to estimate. Diurnal water table fluctuations can be used to directly measure groundwater consumption by phreatophytes, which are often important contributors to the total ET in riparian systems. Although such methods are cost effective, significant uncertainties usually exist, and more accurate techniques continue to be developed. In this study we present a new ''Fourier method'' for calculating daily (and longer) groundwater ET consumption using a moving, multiday sine function to capture robust, diurnal water table fluctuations. The technique is tested and calibrated in Tamarix chinensis and Populus deltoids-dominated riparian areas in the Middle Rio Grande region of New Mexico and in a Phragmites australis-dominated riparian wetland in south-central Nebraska (using independent, energy balance estimates of ET). The results show that-at both field sites-the new Fourier technique performs significantly better than the commonly used White method, regardless of the length of the moving window that is employed. The Fourier method presented here provides a step toward increasing the accuracy of ET estimates from diurnal water table fluctuations. Guidelines are defined for applying the new and improved method in the most accurate fashion, based on groundwater hydrographs and solar radiation data (or theoretical clear-sky estimates).

Abstract.Interactions between shallow groundwater and land surface processes play an important role in the ecohydrology of riparian zones. Some recent land surface models (LSMs) incorporate groundwater-land surface interactions using parameterizations at varying levels of detail. In this paper, we examine the sensitivity of land surface evapotranspiration (ET) to water table depth, soil texture, and two commonly used soil hydraulic parameter datasets using four models with varying levels of complexity. The selected models are Hydrus-1D, which solves the pressure-based Richards equation, the Integrated Biosphere Simulator (IBIS), which simulates interactions among multiple soil layers using a (water-content) variant of the Richards equation, and two forms of a steady-state capillary flux model coupled with a single-bucket soil moisture model. These models are first evaluated using field observations of climate, soil moisture, and groundwater levels at a semi-arid site in south-central Nebraska, USA. All four models are found to compare reasonably well with observations, particularly when the effects of groundwater are included. We then examine the sensitivity of modelled ET to water table depth for various model formulations, node spacings, and soil textures (using soil hydraulic parameter values from two different sources, namely Rawls and Clapp-Hornberger). The results indicate a strong influence of soil texture and water table depth on groundwater contributions to ET. Furthermore, differences in texturespecific, class-averaged soil parameters obtained from the two literature sources lead to large differences in the simulated depth and thickness of the "critical zone" (i.e., the zone within which variations in water table depth strongly Correspondence to: M. E. Soylu (evren@huskers.unl.edu) impact surface ET). Depending on the depth-to-groundwater, this can also lead to large discrepancies in simulated ET (in some cases by more than a factor of two). When the Clapp-Hornberger soil parameter dataset is used, the critical zone becomes significantly deeper, and surface ET rates become much higher, resulting in a stronger influence of deep groundwater on the land surface energy and water balance. In general, we find that the simulated sensitivity of ET to the choice of soil hydraulic parameter dataset is greater than the sensitivity to soil texture defined within each dataset, or even to the choice of model formulation. Thus, our findings underscore the need for future modelling and field-based studies to improve the predictability of groundwater-land surface interactions in numerical models, particularly as it relates to the parameterization of soil hydraulic properties.