Water loss and canopy resistance to water flow from sunflower (Helianthus Annus L) in two different climatic environments

ABSTRACT. In the present paper water loss and variations in canopy resistance in sunflower during kharif and rabi have been analysed. Mean daily water loss of sunflower in rabi season is slightly less than that in kharif. The water loss falls considerably as the soil dries down. The soil water loss is found to be significantly correlated with moisture content in 0-45 cm depth soil profile. The canopy resistance is fairly low when the soil is wet but as soil dries. The resistance increases.

Estimation of Aerodynamic and Canopy Resistances in a Mediterranean Greenhouse Based on Instantaneous Leaf Temperature Measurements

Aerodynamic and canopy resistances have long been considered to be of key interest in model equation parameterization, particularly for the accurate estimation of crop evapotranspiration. However, model parameters applied in greenhouses showed variation affected by the micrometeorological environment. Three experiments were carried out in a plastic greenhouse to evaluate microclimate effects on resistances of a soilless cucumber crop. The regression analysis of canopy-to-air temperature (Tc − Ta) difference on air vapor pressure deficit (VPD) was substituted into the energy balance equation for the estimation of aerodynamic and canopy resistance values. As expected, a fan and pad evaporative cooling system proved to be the more efficient method of decreasing crop temperature (Tc) compared to the forced air ventilation system. The estimated transpiration by the Penman–Monteith model based on calculated aerodynamic and canopy resistance values successfully validated values measured with lysimeters in different growing periods. In this article, we report for the first time the calculation of aerodynamic and canopy resistance values inside a greenhouse based on equations for an open field that were found in the literature. Results may be helpful in Mediterranean greenhouses for direct determinations of plant water evaporative demand and smart climate control systems.

Temporal Upscaling of Rice Evapotranspiration Based on Canopy Resistance in a Water-Saving Irrigated Rice Field

An important element of the hydrologic cycle, the hydrometeorological parameter of evapotranspiration (ET), is critical in the development of effective water resources planning and irrigation scheduling. The ET varies in response to changes in resistance at the canopy surface rc and soil moisture content θ, especially under water-saving irrigation (WSI) practices. Drawing on data collected by eddy covariance in WSI rice paddies in eastern China in 2015 and 2016, variations in ET were studied by calculating and analyzing hourly canopy resistance and daily canopy resistance . Discrepancies were noted between true daily ET with respect to the estimated daily ET at different periods [0700–1600 local time (UTC + 8)]. To estimate in the WSI rice fields, the mean value between 0900 and 1000 LT, and between 1000 and 1100 LT performed considerably better than for a single time. Seasonal estimated ET can be accurately calculated by interpolating at different time intervals, thereby achieving a greater correlation and consistency at 2-day intervals. Then a generalized two-segment line of variation was used to calculate , achieving good results and showing that in the absence of observational data, could be easily calculated through a simplified pattern of variability. In conclusion, an ET temporal upscaling method for a WSI paddy, based on variation in and values, was optimized and is recommended for local application. Future work will focus on temporal upscaling of ET by extrapolating remote sensing instantaneous estimates to daily values.

Canopy Resistance and Estimation of Evapotranspiration above a Humid Cypress Forest

This study presented a two-year data set of sensible heat and water vapor fluxes above a humid subtropical montane Cypress forest, located at 1650 m a.s.l. in northeastern Taiwan. The focuses of this study were to investigate (1) the diurnal and seasonal variations of canopy resistance and fluxes of sensible heat and water vapor above this forest; and (2) the mechanism of why a fixed canopy resistance could work when implementing the Penman–Monteith equation for diurnal hourly evapotranspiration estimation. Our results showed distinct seasonal variations in canopy resistance and water vapor flux, but on the contrary, the sensible heat flux did not change as much as the water vapor flux did with seasons. The seasonal variation patterns of the canopy resistance and water vapor flux were highly coupled with the meteorological factors. Also, the results demonstrated that a constant (fixed) canopy resistance was good enough for estimating the diurnal variation of evapotranspiration using Penman–Monteith equation. We observed a canopy resistance around 190 (s/m) for both the two warm seasons; and canopy resistances were around 670 and 320 (s/m) for the two cool seasons, respectively. In addition, our analytical analyses demonstrated that when the average canopy resistance is higher than 200 (s/m), the Penman–Monteith equation is less sensitive to the change of canopy resistance; hence, a fixed canopy resistance is suitable for the diurnal hourly evapotranspiration estimation. However, this is not the case when the average canopy resistance is less than 100 (s/m), and variable canopy resistances are needed. These two constraints (200 and 100) were obtained based on purely analytical analyses under a moderate meteorological condition (Rn = 600 W·m−2, RH = 60%, Ta = 20°C, U = 2 m·s−1) and a measurement height around two times of the canopy height.

Maize Evapotranspiration Estimation Using Penman-Monteith Equation and Modeling the Bulk Canopy Resistance

Some techniques, such as the Katerji and Perrier approach, estimate the bulk canopy resistance (rc) as a function of meteorological variables and then calculate the hourly evapotranspiration using the Penman–Monteith equation, so that traditional crop coefficients are not needed. As far as we know, there are no published studies regarding using this method for a maize crop. The objective of this study was to calibrate and validate the canopy resistance for an irrigated continuous maize crop in the Midwestern United States (US). In addition, we determined the effect of derivation year, bowen ratio, and the extent of canopy. In this study we derive empirical coefficients necessary to estimate rc for maize, five years (2001–2005) were considered. A split-sample approach was taken, in which each year’s data was taken as a potential calibration data set and validation was accomplished while using the other four years of data. We grouped the data by green leaf area index (GLAI) and the Bowen ratio (β) by parsing the data into a 3 × 3 grouping: LAI (≥2, ≥3, and ≥4) and |β| (≤0.1, ≤0.2, and ≤0.3). The best fit data indicated reasonably good results for all nine groupings, so that the calibration coefficients derived for the conditions LAI ≥ 2 and |β| ≤ 0.3 were taken in light of the longer span associated with LAI ≥ 2 and the larger number of hours. For the calibrations in this subgroup, the results indicate that the annual empirical coefficients for rc are nearly the same and equally effective, regardless of the year used for calibration. Our validation included all the daytime hours regardless of β. Thus, it was concluded that the calibration at our site was independent of the derivation year. Knowledge of the Bowen ratio was useful in calibration, but accurate ET estimates (validation) can be obtained without knowledge of the Bowen ratio. Validation resulted in hourly ET estimates for irrigated maize that explained 83% to 86% of the variation in measured ET with an accuracy of ± 0.2 mm.