Analysis of evaporative flux data for various climates

Estimation of evapotranspiration is a key requirement of hydrologic balance studies and climate analysis. The study reported involved collection of precise weighing lysimeter and meteorological data from three sites representing distinct climates. The combined data set for daily amounts of evapotranspiration and meteorological variables covers a total of 19 years on either an annual or growing season basis. The pan evaporation, Priestley-Taylor, original Penman, and Penman-Monteith evapotranspiration estimating methods arc compared with lysimeter measurements using a moving average of 1-30 days. The results indicate the applicability of the various methods as a function of climate regime and the reduction in standard error of the estimate and increase in the coefficient of determination as a function of length of the moving average period. The results can be used both to determine which methods are most applicable for different climates and the expected magnitude of the error as a function of the estimating interval. This study indicates that a 5-10-day moving average can reduce the standard error of the estimate and increase the coefficient of determination significantly between estimated and measured reference evapotranspiration for several estimating methods for various climates

Evaporation from three water bodies of different sizes and climates: Measurements and scaling analysis

Evaporation from small reservoirs, wetlands, and lakes continues to be a theoretical and practical problem in surface hydrology and micrometeorology because atmospheric flows above such systems can rarely be approximated as stationary and planar-homogeneous with no mean subsidence (hereafter referred to as idealized flow state). Here, the turbulence statistics of temperature(T)and water vapor (q)most pertinent to lake evaporation measurementsover three water bodies differing in climate, thermal inertia and degree of advective conditions are explored. The three systems included Lac Le´man in Switzerland (high thermal inertia, near homogeneous conditions with no appreciable advection due to long upwind fetch), Eshkol reservoir in Israel (intermediate thermal inertia, frequent strong advective conditions) and Tilopozo wetland in Chile (low thermal inertia, frequent but moderate advection). The data analysis focused on how similarity constants for the flux-variance approach, CT/Cq, and relative transport efficiencies RwT/Rwq, are perturbed from unity with increased advection or the active role of temperature. When advection is small and thermal inertia is large, CT/Cq 1)primarily due to the active role of temperature, which is consistent with a large number of studies conducted over bare soil and vegetated surfaces. However, when advection is significantly large, then CT/Cq >1 (orRwT/Rwq < 1). When advection is moderate and thermal inertia is low, then CT/Cq �1. This latter equality, while consistent with Monin–Obukhov similarity theory (MOST), is due to the fact that advection tends to increase CT/Cq above unity while the active role of temperature tends to decrease CT/Cq below unity. A simplified scaling analysis derived from the scalar variance budget equation, explained qualitatively how advection could per¬turb MOST scaling (assumed to represent the idealized flow state)

First passage time statistics of Brownian motion with purely time dependent drift and diffusion

Systems where resource availability approaches a critical threshold are common to many engineering and scientific applications and often necessitate the estimation of first passage time statistics of a Brownian motion (Bm) driven by time-dependent drift and diffusion coefficients. Modeling such systems requires solving the associated Fokker-Planck equation subject to an absorbing barrier. Transitional probabilities are derived via the method of images, whose applicability to time dependent problems is shown to be limited to state-independent drift and diffusion coefficients that only depend on time and are proportional to each other. First passage time statistics, such as the survival probabilities and first passage time densities are obtained analytically. The analysis includes the study of different functional forms of the time dependent drift and diffusion, including power-law time dependence and different periodic drivers. As a case study of these theoretical results, a stochastic model for water availability from surface runoff in snowmelt dominated regions is presented, where both temperature effects and snow-precipitation input are incorporated

Peak grain forecasts for the US High Plains amid withering waters

ACKNOWLEDGMENTS. This paper stems from discussions during the Ettersburg Ecohydrology Workshop in Germany (October 2018), with the corresponding manuscript preparation ensuing in subsequent months. The workshop was funded by the UNIDEL Foundation, Inc. and the University of Delaware. Accordingly, partial support for this paper derived from funding for the workshop. A.M. was supported by the US NSF (Grants NSF-AGS-1644382 and NSF-IOS-175489).Peer reviewedPublisher PD

An experimental investigation of the mean momentum budget inside dense canopies on narrow gentle hilly terrain

Recent theories and model calculations for flows inside canopies on gentle hilly terrain suggest that the impact of advection and pressure perturbations on the mean momentum budget remains problematic when the canopy adjustment length (L-c) is comparable to the hill half-length (L) (referred to as narrow gentle hills). To progress on this problem, detailed laser Doppler anemometry (LDA) and water surface profile measurements were conducted in a large flume simulating a neutrally stratified boundary layer flow over a train of gentle hills covered by a dense canopy with L-c/L similar or equal to 1. The canopy was composed of an array of vertical cylinders with a frontal area index concentrated in the upper third to resemble a tall hardwood forest at maximum leaf area index. The data was presented in terms of component balance of the mean momentum equation decomposed into a background state and a perturbed state induced by topographic variation. We found that the measured and modelled pressure computed from the topographic shape function were not in phase, with the minimum pressure shifted downwind from the hill summit. We also showed that the recirculation region, predicted to occur on the lee side of the hill close to the ground, was sufficiently large to modify the mean streamlines both within the canopy sub-layer and just above the canopy. This adjustment in mean streamlines can be accounted for through an effective ground concept thereby retaining the usability of linear theory to model the mean pressure gradients. The LDA data suggested that the shear stress gradient remained significant at the bottom of the hill in the deeper layers of the canopy and was the leading term balancing the adverse pressure gradient in the recirculation region. The drag force was the leading contributor to the mean momentum balance near the canopy top and within the deeper layers of the canopy at the hill summit. However, we found that the drag force was not the primary term balancing the adverse pressure gradient within the recirculation zone. Advection was not only substantial above the canopy but remained significant in the deeper layers of the canopy near the hill summit as predicted by recent numerical simulations. In short, no one term in the mean momentum balance can be a priori neglected at all positions across a gentle narrow hil