Agriculture

Associated with agriculture are a number of land management problems which can degrade local biotic communities: vegetation clearing, followed by later field abandonment, leaving barren lands; wind erosion and air contamination with particulates coming from fallow fields; disruption of watercourses and fragmentation of floodplains; build-up of saline and alkaline soil crusts; pesticide and herbicide use, affecting numerous non-target organisms; chemical fertilizer (especially nitrate) contamination of streamflows and aquifers, as well as nitrogen enrichment of adjacent wildlands; introduction and spread of exotic weeds, plant diseases and insects; and proliferation of certain wildlife (jackrabbit, cottonrat and passerine birds) beyond what can be supported year-round. On the other hand, agriculture can sometimes have a positive effect on wildlife and fieldside wild plant populations. That typically occurs when farming is practiced on a modest scale, without pesticide and herbicide use (Reichhardt et al. 1994; Rea 1997). A brief trip through the history of Sonoran Desert agriculture illustrates the transition from small-scale to large-scale agriculture.

From the top of "A" Mountain, C.E. Watkins took a photograph overlooking the Tucson basin that illustrates the nature of late 19th century Sonoran Desert agriculture. One can clearly see fields dissected by an elaborate network of canals that diverted surface water from the perennially flowing Santa Cruz river. Save for some dryland agriculture, agriculture was largely absent where the groundwater level was deep, where artesian wells could not be found, and where water could not be diverted from a river. As a result, floodplains had already been highly modified for centuries, but other habitats were largely intact (Bahre 1991). The establishment of such dam and diversion canal projects such as Punta de Agua near San Xavier and the Silver Lake in Tucson in the late 19th century expanded agriculture's conversion of adjacent habitats. However, the effect of such projects was minor compared to the advent of inexpensive, efficient groundwater pumps following World War I (Bahre 1991). This technical advance greatly facilitated the spread of agriculture into virtually all of the non-riparian lowland biotic communities, in the valleys of Arizona and the coastal plains of Sonora.

By the 1940's in Sonora, agricultural water development provided the catalyst for rapid growth rates and extensive vegetation change in the coastal plain. This process began with the appropriation of water from the large aquifers of the Rio Concepci6n, Rio Sonora and Rio Mcitape. Later, the development of dams along the Rio Yaqui and Rio Mayo boosted growth by producing electricity and furthering agricultural development (Búrquez and Martínez-Yrízar 1997). Overall irrigated land area in Sonora nearly doubled between 1950 and 1960 and peaked in 1970 at 3% of the land surface. In Arizona, irrigated land peaked in 1970 at 16% of the state's land surface. In Baja California it peaked in 1960 at 3% of its land surface (Figure 7).

However, it can be argued that the post World War II boom in Sonoran Desert irrigated agriculture has been ecologically and economically unsustainable. Between 1970 and 1980, a 10% decline occurred in the extent of Sonora irrigated land (Figure 7). A host of problems including salinization, increased water pumping costs, and groundwater and soil contamination have dramatically decreased agricultural productivity and left many abandoned, nearly denuded fields. Only 70,000 hectares of the original 150,000 hectares in the Costa de Hermosillo irrigation district remain in production (Búrquez and Martínez-Yrízar 1997). In Arizona, the reduction in irrigated lands statewide was slight between 1970 and 1980 but more marked in most of the Sonoran Desert counties; between 1982 and 1992 cropland declined by 6, 19 and 32% in Pinal, Pima, Madcopa and Yuma Counties, respectively. Between 1987 and 1992, cropland dropped by 8% in Pima County (U.S. Dept. of Commerce 1996). This decrease is due to declining groundwater, increasing pumping costs, decreasing economic subsidies to irrigated agriculture, and the increased value of land for urban development. For example, Tucson and Scottsdale, Arizona are purchasing the agricultural water rights in many adjacent lands to transfer agricultural water use to urban water consumption (Bahre 1991). Since the construction of the Central Arizona Project and the 1980 Arizona Groundwater Management Act, roughly 40,000 hectares of agricultural land has been bought by municipalities, mines, and other industries.

While at a regional scale agriculture has been declining, in certain localities it has remained the same and even increased over the past few decades. Between 1982 and 1992 cropland in the semidesert grassland of Santa Cruz County increased by 12%. It increased by 30% in the La Paz County (Lower Colorado River Valley ) between 1987 and 1992 (U.S. Dept. of Commerce 1996). Likewise, in the Sonoyta Valley of Sonora flanking Organ Pipe Cactus National Monument across the U.S.-Mexico border, cropland quadrupled in extent between 1977 and 1987 due in part to government supported agricultural development. Proximity to U.S. fruit and vegetable markets, inexpensive labor, good quality water, and government agency interest in increased fruit and vegetable acreage in the area mean that agricultural production and the associated descent of groundwater levels could continue in the future (Brown 1991). Some scientists surveyed noted that clearing for agriculture was becoming more severe in portions of the Lower Colorado River Valley, Central Gulf Coast, and Viscaino.

Current Sonoran Desert cropland is most extensive in the border county of Mexicali and the extreme southern end of the Sonoran Desert where most counties have from one-quarter to three quarters of their land surface as cropland. The central section around Hermosillo, Sonora is 15-25% cropland and the rest of the area is less than 15%. However, these figures do not include the millions of hectares of abandoned agricultural land. In Arizona's Casa Grande and Santa Cruz valleys alone there are approximately 325,000 ha of abandoned farmland. Some fields are barren even though they were abandoned 35 years ago, and others support mostly burroweed (Isocoma tenuisecta), and all generally provide very poor wildlife habitat (Jackson 1991). There are some efforts to reseed these areas with exotic grasses (Bahre 1991) which may reduce soil erosion but further threaten the integrity of surrounding native vegetation communities. However, large-scale restoration of abandoned farm lands to native vegetation is possible, depending on prior field management, proximity to native vegetation, soil characteristics, and other factors (Jackson 1991).

From:
Nabhan, Gary Paul and Andrew R. Holdsworth. 1998. State of the Sonoran Desert Biome: Uniqueness, Biodiversity, Threats and the Adequacy of Protection in the Sonoran Bioregion. p.34-36. Tucson, Ariz.: The Wildlands Project.

Sources

• Bahre 1991
• Brown 1991
• Búrquez and Martínez-Yrízar 1997
• Jackson 1991
• Rea 1997
• Reichhardt et al. 1994;
• U.S. Dept. of Commerce 1996