Engineering Commons^™

First Long-Term, Direct Measurements Of Evapotranspiration And Surface Water Balance In The Nebraska Sandhills, David P. Billesbach, Timothy J. Arkebauer

Department of Biological Systems Engineering: Papers and Publications

The Nebraska SandHills is a vast grassland ecoregion that also serves as the main recharge zone for the High Plains (Ogallala) aquifer. Despite the national importance of this region in supplying irrigation water and supporting the US cattle industry, there have, until now, been no long-term, direct measurements of the components of the surface water balance. We have addressed this issue by installing energy balance/Bowen ratio (EBBR) flux towers in three key ecosystems. We report here the results of 7 years of measurements and interpret them in terms of annual weather variability. Additionally, these data have allowed us to make …

Global Estimation Of Evapotranspiration Using A Leaf Area Index-Based Surface Energy And Water Balance Model, H. Yan, S.Q. Wang, David P. Billesbach, Walter Oechel, J.H. Zhang, Tilden Meyers, Ta. Martin, Roser Matamala, Dennis D. Baldocchi, G. Bohrer, D. Dragoni, R. Scott

Department of Biological Systems Engineering: Papers and Publications

Studies of global hydrologic cycles, carbon cycles and climate change are greatly facilitated when global estimates of evapotranspiration (E) are available. We have developed an air-relative-humidity-based two-source (ARTS) E model that simulates the surface energy balance, soil water balance, and environmental constraints on E. It uses remotely sensed leaf area index (L_ai) and surface meteorological data to estimate E by: 1) introducing a simple biophysical model for canopy conductance (G_c), defined as a constant maximum stomatal conductance g_smax of 12.2mm s⁻¹multiplied by air relative humidity (R_{h …}

Net Radiation Dynamics: Performance Of 20 Daily Net Radiation Models As Related To Model Structure And Intricacy In Two Climates, Suat Irmak, Denis Mutiibwa, José O. Payero

Department of Biological Systems Engineering: Papers and Publications

We compared daily net radiation (R_n) estimates from 19 methods with the ASCE‐EWRI R_n estimates in two climates: Clay Center, Nebraska (sub‐humid) and Davis, California (semi‐arid) for the calendar year. The performances of all 20 methods, including the ASCE‐EWRI R_n method, were then evaluated against R_n data measured over a non‐stressed maize canopy during two growing seasons in 2005 and 2006 at Clay Center. Methods differ in terms of inputs, structure, and equation intricacy. Most methods differ in estimating the cloudiness factor, emissivity (α), and calculating net longwave radiation (R_nl). All methods use …

Actual And Reference Evaporative Losses And Surface Coefficients Of A Maize Field During Nongrowing (Dormant) Periods, Christopher H. Hay, Suat Irmak

Department of Biological Systems Engineering: Papers and Publications

Effective water resources planning, allocation, management, and use in agroecosystems require accurate quantification of actual evapotranspiration (ET_c) during growing and nongrowing (dormant) periods. Prediction of ET_c for a variety of vegetation surfaces during the growing season has been researched extensively, but relatively little information exists on evaporative losses during nongrowing periods for different surfaces. The objectives of this research were to evaluate ET_c in relation to available energy, precipitation, and grass and alfalfa-reference ET (ET_o and ET_r) for a maize (Zea mays L.) field and to analyze the dynamics of surface coefficients …

A Modified Surface Energy Balance For Modeling Evapotranspiration And Canopy Resistance, Luis O. Lagos

Department of Biological Systems Engineering: Dissertations and Theses

A modified surface energy balance (SEB) model based on the Shuttleworth-Wallace and Choudhury-Monteith methods was developed to estimate evaporation from soil covered by crop residue, and transpiration from crop canopies. The model describes the energy balance and flux resistances for partially-vegetated and residue-covered surfaces. Physical and biochemical energy storage terms and lateral fluxes are neglected in the model. Net radiation is one of the inputs in the SEB model and provides the energy needed for soil evaporation, crop transpiration and heat transfer through the canopy, soil/residue surfaces and the atmosphere.

A sensitivity analysis of the SEB model parameters showed that …

Variability Analyses Of Alfalfa-Reference To Grass-Reference Evapotranspiration Ratios In Growing And Dormant Seasons, Suat Irmak, Ayse Kilic, T. A. Howell, Derrel Martin, J. O. Payero, K. S. Copeland

Department of Biological Systems Engineering: Papers and Publications

Alfalfa-reference evapotranspiration (ET_r) values sometimes need to be converted to grass-reference ET (ET_o), or vice versa, to enable crop coefficients developed for one reference surface to be used with the other. However, guidelines to make these conversions are lacking. The objectives of this study were to: (1) develop ET_r to ET_o ratios (K_r values) for different climatic regions for the growing season and nongrowing (dormant) seasons; and (2) determine the seasonal behavior of K_r values between the locations and in the same location for different seasons. Monthly average K_r values …