In Chapter 3 a number of methods were presented that can be used to determine the atmospheric fluxes of the surface energy and water balance: sensible and latent heat flux. Those methods were based solely on the use of data regarding wind, temperature and humidity: either the fluctuating parts of the signal (eddy-covariance method) or the mean values: vertical gradients or differences (similarity theory).

In this chapter we not only use our knowledge on the turbulent fluxes, as dealt with in Chapter 3, but also combine it with the energy balance equation (Chapters 1 and 2) and information on the vegetation (Chapter 6). First the Bowen ratio method is discussed in Section 7.1. Next the Penman–Monteith equation that describes the transpiration from vegetation is dealt with in Section 7.2. Finally, simplified estimates for evapo(-transpi)ration are given in Section 7.3 and dewfall (inverse evaporation) is discussed in Section 7.4.

The term “combination methods” in the title of this chapter has two different connotations:

• In general, the term “combination methods” refers to methods that combine the energy balance equation with information on turbulent transfer (all methods in this chapter).

• In a more restricted sense, the term “combination equation” refers to the Penman method (and derived methods) that combines the effects of two factors that determine evaporation (see Farahani et al., 2007 ). These are available energy (represented by the “radiation term”) and atmospheric demand (ability of the atmosphere to remove water vapour, represented by the “aerodynamic term”). In this restricted sense, the term refers only to the methods discussed in Section 7.2.

Preface

This book has its roots in courses on Micrometeorology by Henk de Bruin and courses on Soil Physics and Agrohydrology by Reinder Feddes and colleagues at Wageningen University and Research Centre. Most universities teach these subjects in separate courses. In 2007, during a BSc-education reprogramming round at Wageningen University, micrometeorology, soil physics and agrohydrology were brought together in the current course ‘Atmosphere-Vegetation-Soil Interactions’. As teachers we had our reservations, but it turned out to work very well.

The interface between atmosphere and land is the location where both domains exchange energy, water and carbon. On the one hand, processes in soil and vegetation influence the development in the atmosphere (e.g., cloud formation). On the other hand, the atmospheric conditions determine to a large extent what happens below the soil surface (e.g., through the extraction of water for transpiration). Many environmental challenges, whether they concern climate change in drought-prone areas, salinization of coastal regions, development and spread of plant pathogens, natural vegetation impoverishment due to deep drainage or low water use efficiency in irrigated agriculture, have their origin in close interactions between atmosphere and land. To understand these processes and solve practical problems, students and professionals should have operational knowledge of transport processes in both domains and be able to understand how the atmosphere affects the land and vice versa.

This chapter shows how the knowledge from the previous chapters can be combined to understand and manage processes at the land–atmosphere interface. First, attention is paid to the estimation of crop water requirements using the crop factor method and to the direct measurement of evapotranspiration using lysimeters. Then it is shown how in a semiarid region the water productivity of irrigated crops can be studied and improved. Finally, the response of different vegetation types (grass and forest) to heat wave conditions is studied.

Crop Water Requirements

Evapotranspiration determines to a large extent the hydrological cycle and the environmental conditions near the soil surface. There is a direct relation between the ratio of actual to optimal transpiration and the ratio of actual to optimal crop yield. Irrigation water requirements are determined by the amount of evapotranspiration relative to the amount of natural rainfall and readily available soil moisture. Groundwater recharge and soil salinization also depend largely on the amount of evapotranspiration. In the context of agricultural practice the water required to grow a crop does not only include the water loss due to evapotranspiration, but also the water needed to leach salts and to compensate for nonuniform application of the water (Allen, 1998).

Plants serve as an intermediary between the atmosphere and the soil: they efficiently transport soil moisture into the air and at the same time ingest atmospheric CO2 for their growth. This chapter deals with the transport of water inside plants (from the root to the stomata), the link between water uptake and dry matter production and the modification of the near-surface atmosphere by vegetation, including microclimate, dew and rainfall interception.

Functions of Water in the Plant

Water performs many essential functions within plants. As a chemical agent, water facilitates many chemical reactions, for instance in assimilation and respiration. Water is a solvent and a transporter of salts and assimilates within the plants. Water enables the regulatory system of the plant, as it carries the hormones and substances that are required for plant growth and functioning. Water confers shape and solidity to plant tissues. If the water supply is insufficient, herbaceous plants and plant organs that lack supporting tissue will lose their strength and wilt. The hydrostatic pressure in cells depends on their water content and permits cell enlargement against pressure from outside, which originates either from the tension of the surrounding tissue or from surrounding soil. The large heat capacity of water greatly dampens the daily fluctuations in temperature that a plant leaf may undergo, due to the considerable amount of energy required to raise the temperature of water. Energy is also required to convert liquid water to vapour that transpires from leaves, causing cooling due to evaporation. Without these temperature compensating effects, plants would warm up much more and eventually die from overheating. Owing to these effects, transpiration rates can be estimated from surface temperatures, obtained by infrared thermography using remote sensing from aeroplanes or satellites (Ehlers and Goss, 2003).

Introduction

Compared to the height of the atmosphere, the depth of the ocean and the thickness of Earth’s crust, the permeable soil above the bedrock is an amazingly thin body – typically not much more than a few metres and often less than 1 m. Yet this thin layer of soil is indispensable to sustain terrestrial life. Soil contains a rich mix of mineral particles, organic matter, gases, and soluble compounds. When infused with water, soil constitutes a substrate for the initiation and maintenance of plant and animal life. Precipitation falls intermittently and irregularly, although plants require a continuous supply of water to meet their evaporative demand. The ability of soil to retain soil moisture (and nutrients) is crucial for vegetation to overcome drought periods. Soil determines the fate of rainfall and snowfall reaching the ground surface – whether the water thus received will flow over the land as runoff, causing floods, or percolate downward to the subsurface reservoir called groundwater, which in turn maintains the steady flow of springs and streams. The volume of moisture retained in the soil at any time, though seemingly small, greatly exceeds the volume in all the world’s rivers (Hillel, 1998). Without the soil, rain falling over the continents would run off immediately, producing devastating floods, rather than sustaining stream flow. The normally loose and porous condition of the soil allows plant roots to penetrate and develop within it so as to obtain anchorage and nutrition, and to extract stored moisture during dry spells between rains. But the soil is a leaky reservoir, which loses water downward by seepage and upward by evaporation. Managing the top system in water deficit regions so as to ensure the survival of native vegetation as well as to maximize water productivity by crops requires monitoring the water balance and the consequent change of moisture storage (as well as nutrient storage) in the root zone (Hillel, 1998). Soil regulates the amount of evapotranspiration, which is with rainfall the largest component of the hydrological cycle. In weather prediction, climate and environmental research and groundwater recharge, the amount of evapotranspiration plays a key role. Therefore not only a qualitative understanding of the soil water flow mechanisms is required, but also a precise quantitative knowledge of these processes.

Introduction

This chapter is concerned with exchange processes between the surface and the atmosphere. According to the surface energy balance, during daytime the net input of energy at Earth’s surface (Q* – G) is used to supply heat to the atmosphere and to evaporate water. This heat and water vapour needs to be transported away from the surface. During night time, on the other hand, as we have seen in Chapter 2, the available energy is generally negative and hence the sensible heat is transported downward (water vapour can go either way). The exact partitioning between the sensible and latent heat flux (both during day time and night time) is at this stage not crucial and is dealt with later in Chapter 7.

How does this transport of heat and water vapour from and to the surface occur? If we take heat transport as an example, one option could be to transport the heat by molecular heat diffusion. A typical daytime value for the sensible heat flux could be 100 W m–2, and the thermal diffusivity of air is around 2·10–5 m2 s–1. Then we can derive from Eq. (1.6) that a vertical temperature gradient of more than 4000 K per meter would be required (note that in this case the transported quantity used in Eq. (1.6) is enthalpy per unit volume: ρcpT). It is clear that vertical temperature gradients of this magnitude do not occur, so there must be another mode of transport. This is turbulent transport: heat, water vapour (and other gases) as well as momentum are transported by the movement of parcels of air that carry different concentrations of heat, water vapour, etc.

Introduction

Whereas roughly 70% of Earth’s surface is covered by oceans, the remaining 30% of land has a profound influence on processes in the atmosphere (e.g., differential heating, drag, evaporation and resulting cloud formation, composition of the atmosphere). This impact is due to the large variability in the properties (e.g., albedo, roughness, soil type, land cover type, vegetation cover) and states (e.g., soil moisture availability, snow cover) of the land surface. The processes occurring at the land surface are often grouped under the terms biogeophysical and biogeochemical processes (Levis, 2010): they influence the state and composition of the atmosphere both through physical and chemical processes, and biological processes play an important role in both.

Although the interface between Earth and atmosphere is located at the surface, subsurface processes in the soil are of major importance because part of the energy and water exchanged at the surface is extracted from or stored in the soil. Plants play an important role in extracting water from deeper soil layers and providing it to the atmosphere. In return, processes in the soil and plants (e.g., transport of water, solutes, and energy) are strongly influenced by atmospheric processes (e.g., evaporation and precipitation).

Introduction

At the soil surface, nutrients, pesticides and salts dissolved in water infiltrate the soil. The residence time of these solutes in the vadose zone may have a large effect on soil and groundwater pollution:

• Organic compounds are mainly decomposed in the unsaturated zone, where the main biological activity is concentrated.

• Many plants have no active roots below the groundwater level and therefore extract water and nutrients only from the soil in the unsaturated zone.

• Whereas in the unsaturated zone the transport of solutes is predominantly vertical, in the saturated zone solutes may disperse in any direction, threatening groundwater extractions and surface water systems.

Therefore, to manage soil and water related environmental problems effectively, proper quantification of the transport processes in the unsaturated zone is important (Beltman et al., 1995). For a number of reasons in delta areas relatively much attention is paid to solute transport in soils. In delta areas like the Netherlands the population density is high, the chemical industry is intensive, the agrochemical input in the agriculture is huge, the sedimented soils are very permeable, the groundwater levels are shallow and the groundwater recharge fluxes are large due to the humid climate.

This chapter shows how the methods discussed in the previous chapters are applied in hydrological and meteorological models. The SWAP (Soil, Water, Atmosphere, Plant) model is an example of a field-scale ecohydrological model (Section 9.1). In Section 9.2 various aspects of land–surface models as used in weather and climate models are discussed.

Introduction

SWAP simulates transport of water, solutes and heat in the vadose zone (Kroes et al., 2008; Van Dam et al., 2008). The model includes vegetation growth, as affected by meteorological and hydrological conditions. The upper boundary of the model domain is a plane just above the canopy. The lower boundary corresponds to a plane in the shallow groundwater (Figure 9.1). In this model domain the transport processes are predominantly vertical; therefore SWAP is a one-dimensional, vertical directed model. The flow below the groundwater level may include lateral drainage fluxes, provided that these fluxes can be prescribed with analytical drainage formulas. The model is very flexible with regard to input data at the upper and lower boundaries. At the top general data on rainfall, irrigation and evapotranspiration are used. For frost conditions a simple snow storage module has been implemented and soil water flow will be impeded when soil temperature descends below zero. To facilitate temporal detailed studies on surface runoff and diurnal transpiration fluxes, evapotranspiration and rainfall data can be specified at daily and shorter time intervals. At the model lower boundary, various forms of head and flux based conditions are used.