TOEFL Reading: ETS-TOEFL阅读机经 - N77X2S18V67Q1SQ8M$

After the arrival of hunter-gatherers in the southwestern region of North America, several alternative types of agriculture emerged, all involving different solutions to the Southwest's fundamental problem: how to obtain enough water to grow crops in an environment in which rainfall is so low and unpredictable that little or no farming is practiced there today. People experimented with alternative strategies for almost a thousand years in different locations, and many experiments succeeded for centuries, but eventually all except one succumbed to environmental problems caused by human impact or climate change. One strategy was to live at higher elevations where rainfall was higher, as did the Mogollon, the people at Mesa Verde, and the people of the early agricultural phase at Chaco Canyon known as the Pueblo I phase. But that carried a risk, because it is cooler at high than at low elevations, and in an especially cool year, it might be too cold to grow crops at all. An opposite extreme was to farm at the warmer low elevations, but there the rainfall is insufficient even for dryland agriculture. The Hohokam got around that problem by constructing the most extreme irrigation system in the Americas outside Peru. But irrigation entailed the risk that human digging of ditches and canals could lead to sudden heavy water runoff from rainstorms, digging further down into the ditches and canals and carving out deep channels called arroyos. In that case, the water level would drop below the field level, making irrigation impossible for people without pumps.   A more conservative strategy was to plant crops only in areas with reliable springs and groundwater tables. That was the solution initially adopted by the Mimbres and by people in the phase known as Pueblo II. However, it then became dangerously tempting to expand agriculture during wet decades with favorable growing conditions into marginal areas with less reliable springs and groundwater. The population multiplying in those marginal areas might then find itself unable to grow crops and might starve when the unpredictable climate turned dry again. That fate actually befell the Mimbres, who started by farming the floodplain and then began to farm adjacent land above the floodplain as their population came to exceed the floodplain's capacity to support it. They got away with their gamble during a wet climate phase, when they were able to obtain half their food outside the floodplain. However, when drought conditions returned, that gamble left them with a population double what the floodplain could support, and Mimbres society collapsed suddenly under the stress. Still another solution was to occupy an area only for a few decades, until the area's soil became exhausted, then to move to another area. That method worked when people were living at low population densities, when there were many unoccupied areas to move to, and when each occupied area could be left unoccupied again for sufficiently long after occupation so that its vegetation and soil nutrients had time to recover. However, the method of shifting sites after a short occupation became impossible at high population densities, when people filled up the whole landscape and there was nowhere left empty to move to. One more strategy was to plant crops at many sites even though rainfall was locally unpredictable and then to harvest crops at whichever sites did get enough rain to produce a good harvest and to redistribute some of the harvest to the people still living at all the sites that did not happen to receive enough rain that year. But redistribution was not without risks because it involved a complex political and social system to integrate activities between different sites, so when that complex system collapsed, lots of people ended up starving. The remaining strategy was to plant crops and live near permanent or dependable sources of water, but on landscape benches above the main floodways, so as to avoid the risk of a heavy flood washing out fields and villages, and to practice a diverse economy, exploiting ecologically diverse zones so that each settlement would be self-sufficient. That solution, adopted by people whose descendents live today in the Southwest's Hopi and Zuni villages, has succeeded for more than a thousand years.