Isabel Alves

Modelling surface resistance from climatic variables

For the Penman‐Monteith equation to be used to predict crop evapotranspiration in a one-step approach, methodologies for determining surface resistance (rs) must be available. One usual approach to the modelling of rs is to compute it by inverting the Penman‐Monteith equation and then relate it to the most important environmental variables (radiation, temperature, vapour pressure deficit) using the multiplicative model of Jarvis. In this paper, some results obtained for lettuce are presented to illustrate the pitfalls of this approach. It is shown that the same environmental variables and the same functional forms that are used in the Jarvis model are already considered when calculating rs as the residual term. One cannot thus expect to get a better insight on the behaviour of rs with the multiplicative model. Also, as rs includes information on the transport conditions inside the canopy and thus, is dependent on wind speed (or, indirectly, on the aerodynamic resistance), procedures that only contemplate stomatal functioning may be not adequate. The interactions between rs and latent heat flux are also discussed and indicate that future studies should be focused on the determinism and quantification of the energy partitioning. # 2000 Elsevier Science B.V. All rights reserved.

AERODYNAMIC AND SURFACE RESISTANCES OF COMPLETE COVER CROPS: HOW GOOD IS THE “BIG LEAF”?

The Penman-Monteith equation is based on the assumption that the canopy can be reduced to a “big leaf”. Given the most commonly used formulation of aerodynamic resistance (ra), this “big leaf” is considered to be, implicitly, at the d + zoH level (where d is zero plane displacement height and zoH is roughness length for heat transfer). This can lead to negative values of surface resistance (rs) when the leaves of the top of the canopy (between d + zoH and crop height hc) are the ones that most contribute to total water loss to the atmosphere. To avoid this, ra should be computed from the top of the canopy to the reference height in the atmosphere. Also, one concludes that rs for complete cover crops cannot be computed by simply averaging stomatal resistance since the main condition, the driving force being the same in all of the elements of the “circuit”, is violated.

Non-water-stressed baselines for irrigation scheduling with infrared thermometers: A new approach

Abstract Surface temperature measured with infrared thermometers is an important tool for irrigation scheduling which has been in practice for some decades. Several indices have been developed to time irrigation events. The most useful is the Crop Water Stress Index (CWSI). Its use, however, relies on a non-water-stressed baseline that, although having a theoretical basis, is to be determined experimentally given the uncertainties related to the surface resistance of the crop. The drawbacks of this procedure, besides the non-transferability of the lines from place to place, are that the surface temperature measurements have always to be made under similar weather conditions. A new definition of a non-water-stressed baseline theoretically based and driven by weather variables that can easily be measured and/or estimated is proposed that allows measurements at any time of the day and whatever the weather conditions, thus simplifying the task of the irrigator.

CROP WATER REQUIREMENTS

This article summarizes the essential definitions and methodologies for estimating crop water and irrigation requirements. The concept of reference evapotranspiration is assumed relative to a crop canopy such as grass but with constant crop characteristics. The hypotheses on which this approach is based are discussed relative to crop surface and aerodynamic resistances to heat and vapor fluxes. The crop evapotranspiration is defined using crop coefficients applied to the reference evapotranspiration, which reflect the canopy differences between the crop and the reference crop. Both time-averaged and dual-crop coefficients are explained, the first when the coefficients relative to crop transpiration and evaporation from the soil are summed and averaged for the crop-stage periods, the latter when a daily calculation of transpiration and evaporation coefficients is adopted. Corrections for climate and crop density are presented. Finally, essential information on the soil water balance to estimate crop irrigation requirements is provided.

Evapotranspiration estimation performance of root zone water quality model: evaluation and improvement

Abstract Root zone water quality model (RZWQM) has proven to be useful in evaluating the agricultural systems. However, it has previously been recognized that its evapotranspiration estimates somewhat depart from the measured values indicating the potential for some improvement in this module. Estimation procedure is based on the dual surface approach of Shuttleworth and Wallace, that although having a solid theoretical basis, it is greatly empirical in its application due to the lack of accurate quantitative knowledge of the resistance terms that control the heat fluxes in the canopy. Analysis of the formulation used in RZWQM allowed to detect some weakness in the calculation of bulk surface resistance (rsc), which is based on stomatal resistance averaged by effective leaf area index (LAIeff). In this work, an alteration to the definition of LAIeff is proposed, which leads to an improvement of the estimates. It is also discussed that a greater improvement could be obtained by introducing a model of stomatal resistance response to environmental conditions, which would increase the complexity of the model and be difficult to apply given the lack of adequate knowledge of the quantitative behavior of stomata. Alternatively, the simple crop coefficient approach could be incorporated to define an upper limit to evapotranspiration fluxes and avoid some otherwise unrealistically high estimates of the current version of the model.

EVAPOTRANSPIRATION ESTIMATION FROM INFRARED SURFACE TEMPERATURE. I: THE PERFORMANCE OF THE FLUX EQUATION

In many conditions, there is a need for simple yet accurate methods to estimate crop evapotranspiration. There is interest in the energy balance approach based on infrared surface temperature (T s ). However, the accuracy of the method relies on how closely T s approximates the true aerodynamic temperature (T o ). Several authors have reported a reasonable agreement between the two temperatures, allowing good estimates of latent heat flux. In this study, errors in T o were minimized by measuring the sensible heat flux independently from the other components of the energy balance, and by computing the aerodynamic resistance from the top of the canopy, the level sensed by the infrared thermometer, up to the reference level. Only data gathered in neutral conditions were retained. Results show that T s , especially in dry conditions, can greatly depart from T o . The implications of this difference are that (1) the aerodynamic resistance cannot be corrected for stability conditions based on the difference between radiometric surface and air temperatures; and (2) estimates of sensible heat flux using T s are subjected to considerable errors, either in magnitude and specially in sign. This indicates the need for a different interpretation of the meaning of T s .

EVAPOTRANSPIRATION ESTIMATION FROM INFRARED SURFACE TEMPERATURE. II: THE SURFACE TEMPERATURE AS A WET BULB TEMPERATURE

Infrared surface temperature (T s ) has been considered a practical approximation of the true aerodynamic temperature (T o ). Once shown that the two temperatures are basically not the same, a new interpretation of T s became a necessity. Since T s reflects the evaporative cooling that occurs due to transpiration, the hypothesis that it represents a wet bulb temperature is tested. Mean absolute difference between measured and theoretically derived surface temperature was 0.79°C and maximum relative error less than 10%, which supports the validity of this assumption when dealing with fully watered crops. Surface temperature was then used to estimate short-time crop evapotranspiration using a form of the combination equation first presented by Slatyer and McIlroy (1961). With mean relative errors less than 15%, this approach is fairly accurate and thus can be considered as an alternative to other methods of estimating latent heat flux using radiometric surface temperature.