The Water and Global Change (WATCH) project evaluation of the terrestrial water cycle involves using land surface models and general hydrological models to assess hydrologically important variables including evaporation, soil moisture, and runoff. Such models require meteorological forcing data, and this paper describes the creation of the WATCH Forcing Data for 1958–2001 based on the 40-yr ECMWF Re-Analysis (ERA-40) and for 1901–57 based on reordered reanalysis data. It also discusses and analyses model-independent estimates of reference crop evaporation. Global average annual cumulative reference crop evaporation was selected as a widely adopted measure of potential evapotranspiration. It exhibits no significant trend from 1979 to 2001 although there are significant long-term increases in global average vapor pressure deficit and concurrent significant decreases in global average net radiation and wind speed. The near-constant global average of annual reference crop evaporation in the late twentieth century masks significant decreases in some regions (e.g., the Murray–Darling basin) with significant increases in others.

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Creation of the WATCH Forcing Data and Its Use to Assess Global and Regional Reference Crop Evaporation over Land during the Twentieth Century

The remarkable complexity of soil and its importance to a wide range of ecosystem services presents major challenges to the modeling of soil processes. Although major progress in soil models has occurred in the last decades, models of soil processes remain disjointed between disciplines or ecosystem services, with considerable uncertainty remaining in the quality of predictions and several challenges that remain yet to be addressed. First, there is a need to improve exchange of knowledge and experience among the different disciplines in soil science and to reach out to other Earth science communities. Second, the community needs to develop a new generation of soil models based on a systemic approach comprising relevant physical, chemical, and biological processes to address critical knowledge gaps in our understanding of soil processes and their interactions. Overcoming these challenges will facilitate exchanges between soil modeling and climate, plant, and social science modeling communities. It will allow us to contribute to preserve and improve our assessment of ecosystem services and advance our understanding of climate-change feedback mechanisms, among others, thereby facilitating and strengthening communication among scientific disciplines and society. We review the role of modeling soil processes in quantifying key soil processes that shape ecosystem services, with a focus on provisioning and regulating services. We then identify key challenges in modeling soil processes, including the systematic incorporation of heterogeneity and uncertainty, the integration of data and models, and strategies for effective integration of knowledge on physical, chemical, and biological soil processes. We discuss how the soil modeling community could best interface with modern modeling activities in other disciplines, such as climate, ecology, and plant research, and how to weave novel observation and measurement techniques into soil models. We propose the establishment of an international soil modeling consortium to coherently advance soil modeling activities and foster communication with other Earth science disciplines. Such a consortium should promote soil modeling platforms and data repository for model development, calibration and intercomparison essential for addressing contemporary challenges.

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Modeling Soil Processes: Review, Key Challenges, and New Perspectives

. In the spirit of the Special Issue of HESS to which it contributes, this paper documents the origin and development of the science of natural evaporation from land surfaces over the last 30–35 years, since the symposium A View from the Watershed was held to commemorate the opening of the new Institute of Hydrology (IH) building in 1973. Important subsequent technical progress includes the ability to measure routinely the diurnal cycle of near-surface meteorological variables using automatic weather stations, and of surface energy and momentum exchanges using automated implementations of the Bowen Ratio/Energy Budget technique and the Eddy Correlation technique, along with the capability to estimate the "fetch" for which these measurements apply. These improvements have been complemented by new methods to measure the separate components of evaporation, including: the interception process using randomly relocated below-canopy gauges, transpiration fluxes from individual leaves/shoots using porometers and from plants/plant components using stem-flow gauges and soil evaporation using micro-lysimeters and soil moisture depletion methods. In recent years progress has been made in making theory-based area-average estimates of evaporation using scintillometers, and model-based area-average estimates by assembling many streams of relevant data into Land Data Assimilation Systems. Theoretical progress has been made in extending near-surface turbulence theory to accommodate the effect of the "excess" boundary layer resistance to leaf-to-air transfer of energy and mass fluxes relative to that for momentum, and to allow for observed shortcoming in stability factors in the transition layer immediately above vegetation. Controversy regarding the relative merits of multi-layer model and "big leaf" representations of whole-canopy exchanges has been resolved in favour of the latter approach. Important gaps in the theory of canopy-atmosphere interactions have been filled, including recognising the need, separately, to represent dry-canopy and wet-canopy evaporation in models and the capability to describe wet-to-dry canopy transitions as well as the ability to describe sparse vegetation canopies which only partly cover the underlying soil. There is progress in methods of estimating crop water requirements, but an important recommendation of this paper is that this progress should continue by introducing use of an effective stomatal resistance rather than crop factors. The paper draws attention to relevant theoretical insight on this issue. Progress in theoretical understanding of evaporation processes has been used in the creation of numerous Land Surface Parameterisations (LSPs), the models used to represent land-surface interaction in climate and weather forecast models, and there have been important advances in describing the behaviour of plant stomata in LSPs. A major investment over the last 25 years in conducting Large-Scale Field Experiments, the better to measure, understand and model coupled land-surface/atmosphere interactions, has resulted in improvements in the capabilities of global climate models and the ability of mesoscale meteorological models to describe the enhanced circulation resulting from different forms of land-surface heterogeneity. Progress in understanding why early equations for potential evapotranspiration can be adequate in certain conditions is reviewed. The paper concludes with recommendations for future research.

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Putting the "vap" into evaporation

A recent interpretation of climate model projections concluded that “warmer is more arid.” In contrast, dust records and other evidence have led the geoscience community to conclude that “warmer is less arid” leading to an aridity paradox. The “warmer is more arid” interpretation is based on a projected increase in the vapour pressure deficit (∼ 7–9% K−1) that results in a projected increase in potential evaporation that greatly exceeds the projected increase in precipitation. However, the increase in potential evaporation does not result in an increase in (actual) evaporation which remains more or less constant in the model output. Projected changes in the long-term aridity can be assessed by directly interrogating the climate model output. To that end, we equate lack of precipitation with meteorological aridity and lack of runoff with hydrologic aridity. A third perspective, agro-ecological aridity, is not directly related to the water lost but rather to the carbon gain and is equated with the reduction in photosynthetic uptake of CO2. We reexamine the same climate model output and conclude that “warmer is less arid” from all perspectives and in agreement with the geological records. Future research will need to add the critical regional and seasonal perspectives to the aridity assessments described here.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2015WR017031

On the assessment of aridity with changes in atmospheric CO2

This article provides a focused survey of progress in crop evapotranspiration (ET) measurement and modeling, with particular emphasis on the aspects of interest to the irrigation profession The significant advances in understanding and quantifying crop ET during the past few decades are largely due to our increased ability to measure near-surface climate variables and surface energy and momentum exchanges, complemented by progress in soil and plant sensor technology However, ET measurement is not commonly practiced, and modeling is mostly preferred Much theoretical progress in ET modeling originated with the 1948 work of Penman and the subsequent modification to the Penman-Monteith (P-M) equation and to multi-layer and sparse canopy models These advances strengthened confidence in using the combination equation and encouraged a significant step forward through the adaptation of the P-M equation to provide a standard estimate of reference crop ET for use in the long-established, two-step, crop coefficient (Kc) methodology Recently, there has been a continued progress in this field via the one-step application of the P-M equation to estimate crop ET directly using effective stomatal resistance rather than Kc This article concludes by drawing attention to a general need to improve crop water productivity by reducing non-beneficial soil evaporation and, in this context, the potential value of using improved methods and models to partition ET and to aid scheduling limited irrigation

Evapotranspiration: Progress in Measurement and Modeling in Agriculture

This article provides theoretical analyses that facilitate the use of the Penman-Monteith equation to make a
one-step estimate of crop water requirements. Reluctance to using a one-step estimate results from two outstanding issues,
both of which are addressed. First, no method has been yet defined to handle the problem that meteorological variables are
commonly available only at 2 m above the ground while, when using the Penman-Monteith equation, they are required at some
level above the crop. To resolve this, a blending height is defined in the atmospheric boundary layer (ABL) where
meteorological conditions are independent of the underlying crop. Expressions are derived to calculate the aerodynamic
resistances to, and the vapor pressure deficit at, the blending height from climate variables at 2 m. Consequently, 2 m climate
data can be used in the Penman-Monteith equation, either to estimate transpiration from surface resistance or to calculate
surface resistance from measured transpiration. Second, no table of effective values currently exists for the surface resistance
of different crops equivalent to that for the crop coefficient. This article calls for field studies to address this need. However,
recognizing the need for an interim source of crop-specific surface resistances, a methodology is given for translating the crop
coefficient into equivalent surface resistance. To make this translation, it is necessary to specify the relationship between the
radiative and aerodynamic energy inputs to evapotranspiration when the crop coefficients were calibrated. Finally, a
Penman-Monteith-based, one-step estimation equation is derived that makes proper allowance for the different aerodynamic
characteristics of crops in all conditions of atmospheric aridity, and that estimates crop evaporation for any crop of specified
height from existing crop coefficients using standard 2 m climate data.

W. J. Shuttleworth

Papers

Towards one-step estimation of crop water requirements