Method

 Because energy flow occurs in both the technological and natural components
 of the city and because energy flow per unit of time is a measure of work
 provided, methods for evaluating ecosystem contributions to urban productivity
 center in energy unit measures. But energy occurs in many forms, and it is
 intuitively understood that a Btu of electricity of somehow different from a
 Btu of coal. Similarly, a Btu of tree biomass is different from a Btu of
 sunlight.

 Thus, methodologically, some kind of equivalency must be established among
 different kinds of energy. Establishing this equivalency is fundamental to
 measuring nature's services in energy units. In addition, communication of
 results requires conversion of energy units to dollar units. City planners
 and other interest groups are simply more comfortable planning in dollar units.
 The method of establishing equivalency among energy forms is discussed first,
 followed by the method for conversion to dollar units.

 Equivalency among energy forms is established by evaluating the quantity of
 one kind of energy consumed to "make" another kind. The most straightforward
 method to perform that evaluation is to examine the natural or technological
 system that is designed to convert one kind of energy to another and measure
 the energy consumed for that system. For example, electric power plants make
 electricity from coal andwe know that 3.6 units of coal are consumed to make
 one unit of electricity (Calculation 1, appendix). Solar energy is consumed
 to make organic biomass and we know that, on the average, 100 units of solar
 energy are consumed for each one unit of gross primary production and 200 units
 are consumed for each one unit of net primary production (Calculation 2,
 appendix). Organic biomass is converted or "consumed" to make coal, oil, and
 gas via biological and geological processes. But the conversion efficiency
 in nature has not been measured; thus, the straightforward method cannot be
 used. We do know that the heat content of organic matter is nearly equivalent
 to that of low-grade coal (lignite at 800 Btu's per pound), and that hardwood
 trees have a heat content nearly equivalent to bituminous coal. With that
 knowledge and concepts from theoretical thermodynamics, Odum (1973) estimates
 that a maximum of 5 units of organic matter are consumed to make one unit of
 fossil fuel on the average. We use that maximum in the calculations for
 Capitol Park to obtain a range of values, making our estimates of nature's
 services conservative.

 Next, consider the relationship between energy flow and money. As has been
stressed throughout this paper and shown in Figure 1, the economy and associated
dollar flows depend on two fundamental types of energy: the sun and fossil fuel.
Total annual economic production is measured in dollars as Gross National
Product which in 1975 was $1,437 billion. The GNP measures the value of all
goods and services produced. The energy consumed to produce those goods and
services is the sum of fuel consumption and solar energy consumption. We
have emphasized in this paper that the production of goods and services depends,
not only on fuel consumption, but also on manifestations of solar energy (e.g.,
hydrologic, pollution and climate control). Thus, total energy consumption in
1975 was 99.6 x 1015 (quadrillion) Btu's. This is the sum of 73 quadrillion
Btu's of fuel (Calculation 3, appendix) and 26.6 quadrillion Btu's of solar
energy (Calculation 4, appendix). Thus, in 1975, technology using fuels
contributed about 2/3 of the power behind the GNP and environmental resources
using sunlight contributed the other 1/3. Finally, conversion of energy units