The oceans cover 71% of the earth's surface and cold regions occupy all latitudes north and south above about 65; if we add to the oceans and cold regions the mountainous parts of the world then, together, these environments account for a major part of the globe's surface and yet it is difficult to make measurements in any of them. Deserts and forest, which are either sparsely populated or inhospitable or both, further reduce those parts of the planet that have been measured adequately. This chapter investigates what the problems are and what can be done about them.

The oceans

Sensor compatibility

All the sensors covered under the topic of fresh water measurement (Chapter 10) can also be used in the sea. These include water level staff gauges, float and pressure water level sensors for tidal records (Fig. 13.1), all the water quality sensors (Warner 1972) and current meters. The same sensors that measure atmospheric variables over the land are equally suitable for their measurement over the sea, although there may be problems with how to expose them. Also applicable to use at sea without any modification are data loggers and telemetry systems. So rather than there being a completely different range of instruments for the ocean, it is more a matter of how the instruments are deployed that differs between land and sea applications. There are two main ocean platforms on which instruments are operated – ships and buoys (to which might be added small islands).

The variable

A history of raingauging

The first written reference to rainfall measurement was made by Kautilya in India in his book Arthasastra in the fourth century BC (Shamasastry 1915). The next reference comes from the first century AD in The Mishnah, which records 400 years of Jewish cultural and religious activities in Palestine (Danby 1933). But neither the Indian nor Palestinian measurements continued for long. They were just isolated events, doomed to be ignored and discontinued. There were to be no more quantitative hydrological or meteorological measurements for another 1000 years – a period in which scholars believed, or were forced to believe, that one turned to the sacred scriptures for answers to questions such as ‘Where do springs arise from?’. It was from China, around the year 1247, that the next known reference to quantitative rainfall measurement comes, and during the fifteenth century the practice of measuring rainfall was introduced into Korea, probably from China.

The first raingauge to be operated in Europe was made by the Italian Benedetto Castelli in 1639, a Benedictine monk and student of Galileo. Castelli measured rain only once, using a graduated glass cylinder about 12 cm in diameter and 23 cm deep. He does not seem to have considered doing this on a regular basis. In the 1660s Sir Christopher Wren made the first-known British gauge and later designed a second, which was probably the first ever that used a tipping bucket (Grew 1681).

The variable

Integrated over the whole of its radiation spectrum, the sun emits about 74 million watts of electromagnetic energy per square metre. At the mean distance of the earth from the sun, the energy received from the sun at the outer limits of the earth's atmosphere, at right angles to the solar beam, is about 1353 watts per square metre (W m–2) and is known as the solar constant. In fact the energy received is not quite constant but varies over the year by about 3%, because the earth is in an orbit around the sun that is actually elliptical. The actual output of the sun itself also varies with time, the most familiar regular rhythm being the 11-year sunspot cycle, although the variations due to this are less than 0.1%. There are other, longer, cycles such as the 22-year double sunspot cycle, and the 80–90-year cycles (Burroughs 1994). It is useful to define some terms.

Units and terms

Radiant flux is the amount of electromagnetic energy emitted or received in unit time, usually expressed in watts (1 watt = 1 joule per second).

Radiant flux density is the radiant flux per unit area expressed in watts per square metre (W m–2), although other units such as mW cm–2 and cal cm–2 min–1 may be used (1 W m–2 = 0.1 mW cm–2 = 10–3 kW m–2 = 1.433 × 10–3 cal cm–2 min–1). No longer in wide use is a unit called the langley, which is equivalent to 1 cal cm–2.

It has been shown throughout this book that most measurements of the natural environment are still being made using manual and mechanical instruments developed a century or more ago, albeit refined, but nevertheless limited. In consequence, we are less well informed about the environment than we would like to think, with data of uncertain accuracy and limited geographical coverage.

But with the developments of the past four decades, culminating in intelligent data loggers that record measurements from precise electronic sensors and are able, through satellite telemetry, to transmit their data from anywhere on earth, the environment can now be measured to much higher accuracy, with complete geographical coverage, in near-real time. This potential is only fully achieved, however, if the instruments are of good quality, correctly sited and well maintained. If they are not all these things, the data will be no better than those from the old instruments, possibly worse and certainly unreliable.

As in the past, so it will be in the future that the majority of measurements of the natural environment will continue to be made by individual organisations, small and large, commercial and governmental – research institutes, national weather services, water resources agencies and a host of others. These organisations will continue to buy, develop, operate and maintain equipment of their own choice, to their own standards and to suit their own budgets and purposes, just as they have in the past.

This is but a brief review of a complex subject and is intended to give an overall impression rather than a detailed account. The aim is to clarify what can be sensed remotely, and to put remote sensing into context with ground-based measurements.

What is remote sensing?

Remote sensing (RS) is not the transmission of data from in situ, ground-based sensors at a remote site to a distant base. That is telemetry (Chapter 12).

Whereas in situ measurements are made by sensors in direct contact with the variables they measure, RS measurements are made entirely by sensing the electromagnetic radiation reflected from, or emitted by, the surface of the earth and its atmosphere. Astronomy provides a good example of RS (apart from the spacecraft that have soft-landed on other planets), and photography and, more recently, electronic imaging have become the sensors and recorders of astronomy.

A platform from which RS measurements are made can be simply a mast on the ground, an aircraft, balloon, rocket or spacecraft in orbit around the earth. It is the last that has had the biggest impact on RS and so satellite RS is the main concern of this chapter. However, many of the instruments and techniques used on spacecraft are also used on the other platforms, particularly aircraft. But aircraft are expensive to fly and so are used mainly where an occasional or single measurement is needed and where a local, detailed, low altitude view is necessary.

The variable

Only about 17% of solar radiation is absorbed directly by the atmosphere as it passes through it (Lockwood 1974). In fact the atmosphere is heated primarily as follows: solar radiation heats the ground and the ground's heat is transferred to the air, firstly by molecular diffusion across the laminar boundary layer (a layer only a millimetre or so thick, which clings to most surfaces); beyond this, in the turbulent boundary layer, transfer is by turbulence, which is much more effective at transferring heat than is diffusion. Heat is also, thereafter, transferred by convection, bubbles of warmer air rising into the cooler air above. This transfer of warmed air away from the surface is the sensible heat flux. While it is difficult to measure the rate of energy transfer (see eddy correlation, Chapter 6), the resultant changes in air temperature are important and more easily measured.

A proportion of the heat from the warmed ground is also transferred downwards as the soil heat flux, the rate of transfer being influenced by the amount of water in the soil and also by the pore and particle sizes and the presence of vegetative material. Many biological processes are influenced by soil temperature, from the activity of micro-organisms to the germination of seeds and plant growth, and observations of soil temperature are made at depths down to a metre or more.

The importance of the oceans in weather and climatological processes is now firmly established, and in this the sea-surface temperature is of particular significance. While the measurement of sea temperature is made with identical thermometers to those used for the air, how the observations are made is dealt with in Chapter 13 on the oceans.

Sensors for measurement of the quality and quantity of both surface water, including the oceans, and groundwater are similar in principle, and so it makes for greater clarity if this chapter is organised by sensor type rather than by application.

Measuring water level

Staff gauges

Graduated staff gauges are widely used for the manual measurement of rivers, lakes and sea level. They are usually installed vertically in the river bed or fixed to a weir (Fig. 10.1(a), (b)), bridge or harbour wall. Boards are made in one metre and two metre lengths and are about 15 cm wide, fixed one above the other to cover greater depths, and marked to span up to 12 metres, or more. Alternatively, several may be installed, each progressively higher up the bank of a river if there is no structure to which to fix them and the river is deep and wide. They are graduated in a variety of ways, some every centimetre, others every 10 or 20 centimetres – as in the case of some sea level gauges. Boards are also available for fixing at an angle of 45 or 30 degrees, laid flat on river banks, their markings being stretched to compensate. Most are graduated from bottom to top, but others are made with an inverted scale for situations where levels below a reference point are needed. Gauging boards have the advantage of cheapness and simplicity, although care is needed in reading them.

The variable

Wind is caused by imbalances in the atmosphere due to temperature and pressure differences. The movement of air is an attempt to attain equilibrium but, owing to solar heating, this is never achieved. Although air movement is three dimensional, the horizontal component is usually by far the greater and it is this that is normally meant by the term ‘wind’. However, vertical motion also occurs, both at a small scale near to the ground, as eddies caused by turbulent flow and convection, and on a large scale as a result of solar heating in the tropics, which powers the general circulation of the atmosphere.

For the first 100 metres or so above the ground, wind speed increases approximately logarithmically (Fig. 5.1), but with increasing height the influence of the surface has progressively less effect and at an altitude of somewhere between 500 and 2000 metres, depending on surface roughness and other factors such as latitude, the speed becomes constant and equal to the geostrophic wind (the wind blowing parallel to the isobars). The altitude through which the earth's surface has an influence on the wind is known as the planetary boundary layer.

Wind direction is also affected by altitude. At the top of the planetary boundary layer, the direction is the same as that of the geostrophic wind. But descending through the layer, the wind blows at an increasingly oblique angle across the isobars with a component towards the lower pressure region. Plotted from above, the movement of the line of direction marks out a spiral, known as the Ekman spiral (Lockwood 1974).

Before the development of modern data loggers in the 1960s, the only means of automatically recording measurements of the environment was on paper charts, either mechanically or on electrical strip-chart recorders with electrical sensors. It was the arrival of solid-state electronics, in particular its ability to operate digitally, that enabled computers and data loggers to be developed. Both have greatly enhanced the way in which the natural environment can be measured, indeed they have revolutionised it.

The construction of a data logger

The schematic of Fig. 11.1 shows each main section of a data logger. With the development of large-scale integration on one integrated circuit (IC) chip, and of the microprocessor, many of these functions are now carried out on a single IC, supported by a range of peripheral chips such as serial data communicators, memory access controllers, counters and clocks (Fig. 11.2), although even many of these are now on one single chip. However, to explain the functioning of a logger, it is useful to keep the boxes separate. Indeed they were, in reality, physically separate (until the development of the larger ICs in the 1980s), the first loggers using individual transistors, resistors and capacitors with wires interconnecting them. Today, a small number of ICs, mounted on printed circuit boards, perform all of the functions required – in a reduced space, at reduced cost, with increased reliability and with very low power requirements.

The variable

History

Mining engineers in Italy in the seventeenth century, finding it impossible to draw water by single-stage suction-pump to a height of more than about 10 metres, sought an explanation; this also tied in with the question posed by Aristotle as to whether a vacuum could exist in nature. In 1643, Torricelli performed his classic experiment with mercury in an inverted tube and showed that the reason for the pumping problem was that a vacuum formed over the water if suction continued to be applied beyond the height of 10 metres. The atmosphere has weight, and it is the pressure of this weight that supports the liquid column to a height such that its pressure equals atmospheric pressure. The word barometer was coined from the Greek root baros, meaning weight. Five years later Pascal and Perrier showed that the pressure was less at the top of a mountain than at its base, and it was soon realised that barometric pressure also varied with the weather.

At the same time, Robert Boyle was studying in Italy, reading Galileo's writings (which included references to air pressure), and he became aware of the Torricelli experiment. When he returned to England he brought the idea of the barometer with him, and went on to formulate his theories about the relationship between the pressure, temperature and volume of gases – Boyle's laws.

Subsurface water processes

Soil moisture

Soil moisture (or soil water) refers to the water that occupies the spaces between soil particles. It is at its maximum when the soil is saturated, that is when all the air between the particles is replaced by water but, if the soil can drain, the spaces will normally also contain air, the water then forming a thin film on and between the soil particles, held by capillary attraction. As the soil dries out this film becomes thinner and progressively less easy for plant roots to extract. The water is free to move through the soil, up or down, by gravity and by capillary attraction;it is taken up by plant roots, evaporates at the surface or recharges the groundwater.

However, there is also water present in soil which is not free to move or to be taken up by plants but which may nevertheless be detected during measurement – but not differentiated from the free water, depending on the measurement technique used. This is the water of crystallisation or water of hydration – water that is chemically bound to minerals within the soil such as gypsum (CaSO4 2H2O); water may also be bonded to organic material to varying degrees of strength. In making measurements of soil water content, soils of great variety are encountered and any instrument or measurement method must be able to handle them all, from pure peat or sand to silt and clay or a mix of them, all varying in pore size or having a variety of pore sizes combined, and all varying in the extent of chemically bonded water.

Introduction

The atmosphere and the oceans are joint partners in transporting energy and moisture around the globe. Atmospheric circulation not only transports energy and moisture but also particles of dust. Wind transports dust from the continents to the deep ocean basins, where it may play a significant role in supplying biologically available iron to marine phytoplankton (Martin, 1990). At the present day, biological productivity and biomass are limited by iron deficiency in some areas of the world's oceans (e.g. the Southern Ocean). Increased marine productivity may have occurred during past glacial events due to significantly enhanced continental aridity and aeolian transport. Dust concentrations in ice cores from Greenland and Antarctica show that dust fluxes during glacial periods were approximately five to 20 times higher than during interglacial periods (Thompson & Mosley-Thompson, 1981; Petit et al., 1981). Such changes in aeolian transport can in turn exert a major climate feedback, via drawdown of carbon and significant reduction of atmospheric CO2 levels. Major present-day sources of modern dust can readily be identified from remotely sensed ocean-haze data (see Fig. 3.3, Chapter 3). They include north and northwest Africa, south-east Asia, the Indian subcontinent, Arabia, central America and eastern Europe (especially around the Black Sea).

Records of past variations in dust supply and transport exist not only in ice cores but in marine and lake sediments and in terrestrial sequences of loess. Such records can provide a natural archive of past changes in (a) wind patterns and intensities, and (b) dust source areas. As summarized by Rea (1994), three uniformitarian principles are normally applied to interpret the geological record of aeolian dust supply to the oceans.

Introduction

Magnetotactic bacteria (Fig. 5.1A–E) are micro-organisms which feel the torque of the Earth's magnetic field because they produce strongly magnetic particles of magnetite or greigite within their cells. They have been identified in diverse sedimentary environments including lacustrine, brackish, and marine sediments, rivers, saltmarshes, ponds, terrestrial soils and also within stratified ocean waters (Table 5.1). The magnetic particles formed by magnetotactic bacteria may contribute significantly to the magnetic properties of sediments. If, upon death and lysis of the bacteria, the magnetic particles survive burial and become magnetofossils, they may play a significant role in the magnetic record of Quaternary and older sediments. They may thus encode information about the Earth's magnetic field direction and intensity, and the environment at the time of their existence.

Early studies on the physiology of magnetotactic species such as Magnetospirillum magnetotacticum (Blakemore et al., 1979, 1984) and Bifidococcus magnetotacticum (Moench, 1989) suggested that all magnetotactic bacteria are microaerobic (i.e. that they require small amounts of molecular oxygen for their metabolism). This prompted some workers to suggest that their occurrence in extant sediments, and their magnetofossils in ancient sediments, could be used as an indicator of microaerobic conditions (Kirschvink & Chang, 1984; Rhoads et al., 1991). However, we now know that there are several different physiological types of magnetotactic bacteria, including strict anaerobes (species which can only grow in the absence of oxygen). Thus, bacterial magnetite may contribute to the magnetic properties of both oxic and anoxic sediments.