Page 260 - Encyclopedia Of World History Vol V

water management 2037

of sweet water in the form of rain and snow. After precip- the life of the green plant is to dissolve nutrients (from the
itation hits the ground it either runs off on the surface, as soil) and transport them into the plant. However, water
rivers and other streams, or soaks into the soil. From the can also dissolve poisons (including salts). Fourth, water
point of view of a farmer cultivating fields in a fixed place, can carry many solid particles in suspension. Erosion of
the water environment is sweet, with water running on the the landscape by running water transports such solid par-
surface, water in the top layer of the soil (soil water), and ticles downstream.
water deeper underground (groundwater). The farmer wants sweet water for his plants in the right
As is true for most life processes, an ideal amount of amounts, at the right times, moving at the right speeds,
water exists for the farmer. However, the environmental and containing the right components (dissolved, sus-
supply of water is variable, fluctuating from place to place pended). Irrigation is the technology that increases the
and from time to time on daily, monthly, and yearly water supply to plants, and drainage is the technology
schedules. Combining the need for an ideal amount of that reduces the water supply to plants.
water with the unending fluctuation yields three kinds of The major energy sources for moving water are grav-
conditions: about right, too much, and too little. “About ity, human muscle, animal muscle, wind, and heat
right” rarely occurs. engines (mechanisms, such as internal combustion
Water has several physical characteristics that are rel- engines, that convert heat energy into mechanical or elec-
evant for managing a farm. First, water is liquid and flows trical energy). For irrigation we supply water at a high
easily. Thus, all water responds to gravity and naturally point, and for drainage we remove it at a low point.
flows downhill. It always takes the shortest possible
route. Water is easy to move if a downhill path can be Irrigation
found. However, because water is a liquid, if a person The elements of a canal irrigation system are an intake,
wants to lift it, a sealed container must be used. Second, where ditches tap into a water source; a main canal
water is heavy. Lifting it requires not only a sealed con- (sometimes many kilometers long); and a series of branch
tainer, but also considerable energy. Third, water is an canals that delivers water to the farmer’s fields. Movable
excellent solvent. One of the major functions of water in gates may control water flow into the branch canals, or
the system may be designed so
that water flows in every chan-
nel simultaneously.
The ditches move the water
from the source to the fields.
The technology for digging
these ditches is simple and has
been invented virtually every-
where. People use digging

A dam at Simatai in
March 2004, a Great
Wall tourist location to
the far north of Beijing.
A fine example of the
Chinese commitment to
dam-building.