Beginning with a discussion of evaporation thermodynamics, it goes on to discussing factors controlling evaporation, evporation estimation, models of evapotranspiration such as Penman, Penman-Monteith, FAO, Hargreaves, Priestley-Taylor, Blaney–Criddle, Turc, and Thornthwaite, selection of a method, estimation of net radiation, measurement of evaporation, potential evapotranspiration, reference and actual evapotranspiration, and estimation of actual evapotranspiration.

Acclimatization and adaptation are two responses of organisms to a changing environment. Each species functions best in a particular thermal environment, and has adaptations that allow it to survive and reproduce successfully in that environment. The range of thermal conditions tolerated will vary with each species. Animals and plants face somewhat different challenges, as plants cannot move to a new environment, but there is also considerable overlap in how animals and plants deal with a variable thermal environment. Organisms must balance their internal water and solute concentrations within tolerable levels. Terrestrial species in dry environments have adaptations for procuring and conserving water, while aquatic species are more likely to be challenged to maintain the proper solute concentration. For many species, the problem of temperature regulation and water regulation are functionally linked, and there may be tradeoffs in how these two critical processes interact. Environmental temperature, water availability, and solute concentration influence species distribution and abundance. Climate change caused by human actions is already significantly affecting species distribution and abundance and ecosystem functioning.

We experience weather in the air around us, and that implies climate: the statistics of weather. But if we are to understand climate, and especially how it changes, we need to examine all its interactions with the planet. In the previous chapter we looked at the role of sea-surface temperature. If we are to go further, we need to look at other components of our planet and their atmospheric interactions. When we see the range of interconnections, we realise that we are dealing with a complex system, with multiple interactions between components. Whenever we see such a system, we must realise the potential for one interaction between components to end up impacting back on another: this is feedback. If we are to understand climate, we must understand these feedbacks. In the last sections we will illustrate one of the most important of these processes with a ‘relatively simple’ model.

Abstract: Hydrological sciences cover a wide variety of water-driven processes at the Earth’s surface, above, and below it. Data assimilation techniques in hydrology have developed over the years along many quite independent paths, following not only different data availabilities but also a plethora of problem-specific model structures. Most hydrologic problems that are addressed through data assimilation, however, share some distinct peculiarities: scarce or indirect observation of most important state variables (soil moisture, river discharge, groundwater level, to name a few), incomplete or conceptual modelling, extreme spatial heterogeneity, and uncertainty of controlling physical parameters. On the other side, adoption of simplified and scale-specific models allows for substantial problem reduction that partially compensates these difficulties, opening the path to the assimilation of very indirect observations (e.g. from satellite remote sensing) and efficient model inversion for parameter estimation. This chapter illustrates the peculiarities of data assimilation for state estimation and model inversion in hydrology, with reference to a number of representative applications. Sequential ensemble filters and variational methods are recognised to be the most common choices in hydrologic data assimilation, and the motivations for these choices are also discussed, with several examples.

The processes by which forests influence large-scale climate are well known, but the specific response to changes in forest cover varies with background climate (e.g., tropical, temperate, boreal), the extent of forest change and type of conversion, and time of year, and differs between day and night. Forests remove carbon dioxide from the atmosphere, thereby lessening planetary warming. Forests also affect climate through exchanges of energy, water, and momentum with the atmosphere, which can warm or cool the climate depending on geographic location, time of year, and time of day. There is a distinct latitudinal pattern from tropical to temperate to boreal forests, with different influences on temperature and different underlying mechanisms. In general, forests cool the daytime surface climate during the growing season through evapotranspiration and other non-radiative processes, and they warm the nighttime climate. Outside of the growing season, forests are generally warmer than open fields, especially in locations where snow is present. Further climate influences occur through chemical emissions that produce aerosols. Much of this understanding comes from models of Earth’s climate.

This chapter delves into the science of forests and streamflow. The amount of water that runs off the land into a stream is known as water yield. It is the water available for human uses. Evapotranspiration is a loss of water that reduces streamflow. Forests increase annual evapotranspiration and reduce annual streamflow compared with grasslands and other types of vegetation. The science, however, is not precise and our understanding is more qualitative than quantitative. Water yield and the climate services of forests represent conflicting demands for water. Forests cool the surface climate through evapotranspiration and remove carbon dioxide from the atmosphere during growth, but in doing so they consume water and reduce water yield for human usage. Consideration of spatial scale further muddies the policy implications because any precipitation benefits of forests occur at large spatial scales covering vast regions of land or entire continents, while the water cost of forests is felt at the scale of the watersheds that supply towns and cities.

The period from the mid-nineteenth century to the end of the century saw heightened interest in the harmful effects of deforestation on rain. There was growing fear that deforestation was turning prosperous lands into deserts, accompanied by efforts to conserve remaining forests or replant denuded lands. In India, Australia, New Zealand, South Africa, Russia, and the United States, the cry was raised: forests must be protected and managed for rain. These efforts followed prior concern in France and British island possessions. In this, forest conservationists advanced a need for government control of forests and used forest influences on rainfall as justification. Opponents, in turn, attacked the premise of forest influences on rainfall and decried the lack of evidence for climate deterioration. The ensuing debate was a narrative of misunderstanding, misuse of data, and hyperbole. It is this aspect of the forest–climate question, the so-called desiccation theory and its misuse to achieve policy goals, that has formed the historiography of the controversy, but beneath the rhetoric is found a fledgling knowledge of forest influences on climate that can be seen in today’s science.

Energy transfers between the biosphere and atmosphere affect climate. These include radiative exchange, seen, for example, in the warmth of sunlight; convection, experienced in the cooling influence of a breeze; evaporation; and conduction of heat in the soil. These fluxes depend on the biophysical and physiological characteristics of plants. In addition, plants remove carbon dioxide from the atmosphere during their growth and emit many chemical species to the atmosphere. Biometeorology is a field of science that studies the interactions between the biosphere and the atmosphere through energy, water, and chemical exchanges. These are regulated by the amount of water in the soil, and so the hydrologic cycle is closely coupled to the energy and chemical cycles. Seasonal changes in leaves, both foliar chemistry and the amount of leaves in the canopy, drive short-term variation in energy, water, and chemical fluxes. The growth of vegetation, accumulation of carbon in plant biomass and soil, and changes in floristic composition cause longer-term changes in biosphere-atmosphere coupling over periods of decades, centuries, and millennia.

Different types of crops require different types of climate and soil. Further, different crops and their optimum production have different irrigation requirements with respect to the frequency of irrigation, timing of irrigation, and amount of irrigation water per irrigation. The objective of this chapter is to briefly discuss the types of crops and their water requirements.

Evapotranspiration (ET) is either estimated for a reference crop, such as alfalfa or grass, or as a potential ET value. The estimated ET value is then modified for a specific crop using a crop coefficient, which is what constitutes the crop water use or consumptive use. Estimation of the crop coefficient is therefore fundamental. The objective of this chapter is to discuss the methods for converting the estimated ET to a specific crop ET, which is the same as crop water use, or to compute the crop coefficient.

Irrigation scheduling is a fundamental component of irrigation management and is vital for optimum agricultural production. It varies with the type of crop, soil, climate, method of irrigation, and agricultural practices. There are different methods for irrigation scheduling. This chapter discusses some of the commonly used methods of irrigation scheduling.

This chapter discusses both the role of irrigation in river basin development and closure and how its share in total water use can be reduced. It first briefly outlines the importance of unchecked irrigation development in the growing share of water consumption and the closure process of the basins examined in this volume. This understanding of how irrigation came to play a peculiar role in river basin development is important for discussing how its share can be reduced. The chapter recalls the diversity of policy options available to respond to imbalances between supply and demand and that supply augmentation is generally favored. Finally, the chapter focuses on the issue of "water savings," documenting various responses by the irrigated sector to shortages and exploring how policies to modernize irrigation technologies may inadvertently contribute to enhancing evapotranspiration and therefore undermine purported conservation objectives.