Droughts have been recorded as a problem in Texas since Spaniards explored the area. Álvar Núñez Cabeza de Vaca found a population of soil tillers near the site of present-day Presidio, where it had not rained for two years. Regarding the White man as a god, they begged him to tell the sky to rain. In 1720 a summer dry spell in Coahuila killed 3,500 of the 4,000 horses that the Marqués de Aguayo, governor of Texas, was prepared to bring to Texas. A drought in Central Texas dried up the San Gabriel River in 1756, forcing the abandonment of a settlement of missionaries and Indians. Stephen F. Austin's first colonists also were hurt by drought. In 1822 their initial food crop of corn died from lack of moisture. Each decade since then has been marked by at least one period of severe drought. Associated with dry times are grasshopper plagues, brush and grass fires, sand and dust storms, crop failures and depression, livestock deaths, disease resulting from insufficient and impure drinking water, and migrations of citizens from parched territory. Information concerning pioneer-day droughts is sketchy because of the absence of official statistics; but data on some droughts, especially those during the nineteenth century, can be compiled from individual complaints recorded in newspapers, diaries, and memoirs. In 1883 Texas opened its western school lands, drawing thousands of immigrant farmers to the area. One of the worst droughts in Texas history occurred in 1884–86, causing most of the farmers to fail and to return to the East. In later years official detailed recordkeeping makes possible a better understanding of the geographical distribution of droughts. Drought occurs when an area receives, in a given year, less than 75 percent of its average rainfall. The number of drought years in each of ten geographical areas of Texas in the 100 years between 1892 and 1992 was as follows: Trans-Pecos, sixteen years; lower Rio Grande valley, seventeen; Edwards Plateau, seventeen; South Central, fifteen; Southern, fifteen; North Central, twelve; Upper Coast, thirteen; East Texas, ten; High Plains, ten; and Low Rolling Plains, eight. There has been at least one serious drought in some part of the state every decade of the twentieth century. The most catastrophic one affected every part of the state in the first two thirds of the 1950s. It began in the late spring of 1949 in the lower valley, affected the western portions of the state by fall, and covered nearly all Texas by the summer of 1951. By the end of 1952 the water shortage was critical; Lake Dallas for instance held only 11 percent of its capacity. Spring rains in 1953 gave some brief respite to Northeast Texas. In the Trans-Pecos, however, only eight inches of rain fell the entire year of 1953, and the drought grew worse from 1954 to 1956. Streams only trickled or dried up completely. The drought ended abruptly in the spring of 1956 throughout Texas with slow soaking rains. There were several less severe and shorter droughts in the 1970s. Most were ended by rain from tropical storms. A massive heat wave in 1980 started a severe drought that blistered most of Texas during the early 1980s. This gradually worsened until it reached extreme proportions in the Pecos River valley during 1983. Even mesquite trees withered. It was ended in the western half of the state by the residue of a north Pacific cyclone, which moved across Mexico. The drought shifted eastward in 1984, inflicting hardship on central and southern Texas; some towns ran out of water and others enforced rationing. On occasion, attempts to make rain artificially have been instituted by both private individuals and public organizations, but these have met with little success. Constant improvement in moisture conservation and utilization, however, has aided Texans in their struggle with drought.