DlAGNOSlNG AND MODELING WATER QUALITY

DlAGNOSlNG AND MODELING WATER QUALITY PARAMETERS OF THE YAQUl VALLEY'S AQUIFER IN NORTHWEST MEXICO FOR SALlNlTY RlSK EVALUATlON Juan Manuel cortés-~iménez',Enrique ~ r o y o - ~ i é ~ u eBernardo z ~ * , ~urillo-~mado?, José Luis ~ a r c í a - ~ e r n á n d eJaime z ~ , ~ a r a t u z a - ~ a y á n hSang d Suh ~ e e ~

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Centro de Investigación Regional del Noroeste INIFAP-CIRNO. Norman E Bourlaug, Km.12.5 Col. Valle del Yaqui. Cd. Obregón, Sonora. 85000 México 2 ~ e n t r de o Investigaciones Biológicas del Noroeste (CIBNOR, S.C.), Ap. Postal 128, La Paz BCS, 23090 México "stituto Tecnológico de Sonora. 5 de Febrero 818 Sur, Col. Centro, Cd. Obregón, Sonora. 85000 México 4 ~ u r aResearch l Institute KARICO. 1031-7 Sa-dong Sangrok-gu, Ansan-si, Gyeonggi-do, 426-170, Korea

SUMMARY

I

Salination, a geochemical process related to the buildup of salts in soil and groundwater, affects the agro-ecosystems, reduces the quality of soil, limits the potential uses of groundwater, and is one of the main problems related with degradation of irrigated cropland. In the northwest region of Mexico, the Yaqui Valley is the main agricultural area with 250,000 ha of irrigated cropland. In the historical context of the 'Green Revolution', this semiarid valley, where the 'improved variety-based agriculture' episode originated, used to be a productive agricultura1 district once flourishing with grain fields, but now vast rows of wheat farmland remain unplanted since 1997, due to a drastic reduction of the water storage in the reservoir system built on the Yaqui River basin. An option that temporarily solves the water shortage in this reservoir system consists in the development of a deep-well network, by which 600 million cubic meters of groundwater can be extracted each year. Nevertheless, recent studies reported that the extracted water is classified as one with high salinity or very high salinity. A strategic approach for sustainable soil and water management became necessary to cope with this problem. The objective of this study was to investigate the spatial distribution of soil clay content and water quality extracted from wells in the Yaqui Valley's aquifer through GIS methods. W e estimated an average soil clay content of 54.52%. Salinity is considered to be one of the principal degradation risk problems in this kind of soils. The extracted water from the wells showed an average electrical conductivity (EC) of 2.1 dS m-'. Only 7.1% of the extracted water from wells had an electrical conductivity lower than 0.7 dS m-'. In 73.4% of the cases, the EC of water extracted from wells varied from 0.7 to 3.0 dS m-', while in 19.5% of the wells it evidenced an EC higher than 3.0 dS m-'. In most cases, chloride was the dominant anion, while sodium was the cation with the highest concentration. We identified the

aquifer zones where water of low quality and clayey soils exist. Extracted water was found to be appropriate to irrigate moderately salt-tolerant and tolerant crops, hence, feasibility studies and following actions need to be performed to avoid irreversible degradation of the clayish soils irrigated with the available low-quality water from the aquifer. For this vast agricultura1 valley, salination, as one of the main factors associated with vulnerability, was defined as the degree to which human and environmental systems are likely to experience damage due to a perturbation, or to a stress caused by salinity associated to water depletion, becoming in recent years a central focus of global climatic change and arid-zone agricultural sustainability studies.

KEYWORDS: salinity, Yaqui Valley, SIG, water quality, aquifer, groundwater.

Water has been a major factor in the rise and fa11 of great civilizations through the history. When water was improperly managed, or the hydrological supplies failed, the civilization collapsed. Irrigation projects planned and executed unwisely result in large quantities of salts and other chemical compounds' accumulation in soil and water. Mismanaged irrigation schemes lead to the "sterilization" of part of the best and most productive croplands around the world. Salination of fertile croplands is a persistent problem, wherever irrigation is practiced on a large scale [l], and is considered one of the main problems related with the degradation of cropland soils under irrigation [2].

In Mexico, only 12% of land area is arable, of which less than 3% is irrigated. These proportions couple with a general lack of economic opportunity in rural areas. This situation makes it difficult to raise the productivity and living standards of Mexico's subsistence farmers. Agriculture accounted Sor 4% of GDP in 2002 Sor al1 Mexico, yet agricultural employment accounted Sor over 20% of total employment [3]. In the northwest region of Mexico, the Yaqui Valley is the main agricultural area with 250,000 ha of irrigated cropland. The agricultural activity of the valley depends on the storage of water in the reservoir system built at the Yaqui River catchment basin. Nevertheless, since 1997 there has been a drastic reduction in the water storage in this reservoir system, registering an average storage leve1 of 20% of the design storage capacity [4]. For the 2004 season, there was no water available for agricultural use from the main dam. This phenomenon has originated a significant change in the Yaqui Valley's agricultural practices, changing from cereal-legume rotation system (two crops per year) to a monoculture system, where wheat is the prevailing crop. This situation affects the regional economy, because when only one crop is produced in winter for marketing, in consequence, practically there will be no agricultural activity in summer. An option to solve the water shortage in the reservoir system consists in the development of a deep-well network, by which 600 million cubic meters of groundwater can be extracted each year. However, recent studies stated that the extracted water is classified as that with high salinity or very high salinity (C3 and C4), with an electric conductivity varying from 240 to 6,690 micromhos cm-' and with mid-levels of sodicity [4]. Previous information coincides with the data which indicated that the majority of the available water from the aquifer contains total dissolved solids (TDS) from 1000 to 5000 mg/L [S]. The Yaqui Valley in Northwest Mexico is the birthplace of the so-called 'Green Revolution' for breed wheat, and one of Mexico's most productive agricultural areas. Today, population growth, urbanization, agricultural intensification, land-use change, surface water diversions, groundwater extraction, man-made coastal modifications, wetland conversions, and growing aquaculture threaten the sustainability of the region's natural resources. Water quality

Irrigation water quality is determined by the use of several methods, including the degree of acidity or alkalinity (pH), electrical conductivity (EC), residual sodium carbonate (RSC), and sodium adsorption ratio (SAR). Commonly, an irrigation water quality study is focused on the kind and amount of salts present in the water, and their effects on crop growth and yield. Salts are present in variable concentrations in al1 types of waters, and break down into ions when they dissolve in water. The principal ions produced when salts are dissolved in water, are calcium, magnesium,

sodium, potassium, sulfate, chloride, carbonate, and bicarbonate [6-S]. Salinity

Salinity is considered to be one of the principal degradation risk problems of agricultural soils. Fine texture (clayey soils) has been identified as one of the soil factors that increase the risk of salination and sodification of agrcultural soils, mainly described as Vertisols and Regosols i6,91. Salinity problems are found mainly in the expansibleclay soils [lo]. In the Yaqui Valley of Mexico, it has been observed that salinity in the groundwater shows a variation associated with the clay contents, where montmorillonite and calcite have been identified as the principal components of this clay-silt stratum [ l l , 121. The analysis also indicated that water holds a potential capacity to dissolve the unsaturated minerals, with a consequent potential to increase the aquifer salinity. As a management strategy in this agricultural district, from autumn through winter the well and reservoir waters are mixed, a procedure which lowers the salinity and thus improves the irrigation water quality, while in summer, the water from wells is used without mixing with the reservoir water, for the irrigation of cotton, corn, alfalfa, and citrus. Nevertheless, from autumn 2003 to summer 2004, when there was no water availability for agricultural use from the main regional darn, the water extracted from wells was used without mixing with surface waters to irrigate 65,000 ha. According to the available water quality information, any highly salinized groundwater can be used only for irrigation of soils with a good permeability. However, the Yaqui Valley's predominant soils are Vertisols and Huvisols; accordingly, Vertisols are characterized by a content of 30 percent or more by mass in clayey particles within al1 horizons (layers) of the upper half-meter of the soil profile. These soils also are typified by rampant cracks of at least 1 cm (0.4 inch) wide extending downward from the soil surface. On the other hand, soils texturally classified as 'clays' have a limited interna1 drainage and a low permeability. In this case, farmers and authorities must be aware of a high risk of salination of the soils, under the consideration that they are exclusively irrigated with saline water extracted from the aquifer, Sor which it is quite indispensable to design an appropriate strategy to facilitate the most adequate use for this resource without a negative impact on soils of the cropland. In this sense, there is a need to develop policies and decision-making tools in agriculture on a geographic scale, for which GIS has demonstrated its utility to diagnose and evaluate the risk of environmental damage [l3, 141. As a strategy Sor sustainable soil and water management and in order to reach a sustainable use of soil and water resources in this valuable agricultural area, it is necessary to evaluate the risk of salination and sodification of

soils as a consequence of an indiscriminate use of the salinized water from the aquifer. In this context, the analysis of chemical characteristics of water and soil-textura1 properties was the first step to reach such purpose. The objective of this study was to investigate the clay content of the soils, the water quality parameters, and the spatial distribution of the quality of water extracted from wells wide-spread along the Yaqui Valley's aquifer in the State of Sonora in Northwest Mexico, and to propose an alternative index for water quality modeling.

adsorption ratio' (SAR = ~ a + l d ( C a++ ~ ~ + ' ) / 2 )Soil . clay content in the study zone was determined from 93 soil samples, according to Bouyoucos hydrometer procedure described in the Mexican Official Norms [18], based on international standards.

MATERIALS AND METHODS Study area

The Yaqui Valley is located between the 26" 53' and 28" 37' N, and 108" 53' and 110" 37' W (Fig. l), an arid region of Northwest Mexico where relatively large, irrigated wheat farms are established. It is situated in the northern portion of the west coast of mainland Mexico, limiting the east of the Gulf of California (also called the Sea of Cortes). The dominant soils in this valley, similarly as other soils of arid zones, contain large amounts of soluble alkaline salts that would be removed by solution in a more humid area. Eventually, some of these salts are dissolved by the water that runs off during seasonal floods [lS].

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FIGURE 1 Geographical position of the Yaqui Valley, in Northwest Mexico.

Cartography, GIS, and mapping techniques

Water analysis and diagnosis

Analyses and diagnoses of water quality from the aquifer were performed during 2004-2005 at the Soil Laboratory of the Yaqui Valley Experimental Station (CEVY). Chemical composition of 225 water sarnples from the wells in the Yaqui Valley was analywd [4, 161. For this purpose, the obtained data were geo-referenced and analyzed by means of interpolation procedures. Water samples were characterized by standard methods [6], and quality was determined by well-known criterion [7, 171. Variables analyzed were the main indicators, such as pH and the electrical conductivity (EC), the cations calcium (Ca"), magnesium ( M ~ + ~sodium ), ( ~ a " ) , the anions sulfate ( s 0 i 2 ) , chloride (~1-'),carbonate (CO?), bicarbonate (HCOY~),and the indicator of the sodification known as the index' sodiurn

For the elaboration of maps useful to describe the spatial distribution of the water quality and the soil clay content, we carried out interpolations through the program IDRISI version 4.0 [19]. Other similar programs or systems have proven to be useful in geographic information system (GIS). There are two Florida case studies that demonstrated a GIS-based (GWRAPPS) model's ability to characterize irrigation needs based on spatially heterogeneous soil and climate data, in contrast to a spatially lumped model [20]. Climate parameters in the study zone are shown in Table 1, which are typical of an arid zone. According to the obtained weather information, agriculture is impossible without proper irrigation in this region.

TABLE 1 - Climate parameters in the Yaqui Valley agricultural zone in Northwest Mexico. CLIMATE MONTH PARAMETERS JAN FEB MAR APR MAY JUN JUL AUG Temperature, "C Minimum 0.0 1.0 3.4 5.6 7.2 12.0 18.0 19.4 Lowest mean 12.5 13.1 14.7 17.4 21.0 25.7 28.1 28.1 Mean 17.5 17.6 19.5 22.9 24.4 27.4 30.8 31.8 Highest mean 23.7 25.2 27.2 31.2 34.1 36.0 36.0 35.6 Maximum 35 34.2 35.5 40.3 41.6 42.8 43.6 44.2 Precipitation, mm 19.7 9.7 4.0 2.1 0.6 3.3 58.2 74.5 Evaporation, mm 76.4 93.1 137.5 193.3 247.0 267.6 231.8 210.2 RH, % 75.0 73.0 70.0 58.5 54.0 62.0 73.0 77.0 Average RH (relative humiditv) = 36; preciuitation = 45; evaporation = 20; - vears: , Total annual: precipitation = 280 mm; evaporation = 2005 mm; Monthly mean temperature = 23.5 "C. A

*

1 SEP

OCT

NOV

DEC

14.6 27.3 29.3 35.8 44.0 45.1 184.6 74.0

8.2 23.2 24.9 34.0 41.6 28.5 164.0 66.5

3.6 17.3 20.5 29.1 37.4 10.1 117.5 65.0

-1.8 14.1 15.4 24.6 32.5 21.1 81.3 73.0

RESULTS AND DlSCUSSlON Soil clay content

It was found that the soil has a high clay content (Fig. 2), and minimum value was 42.5%, maximum 73.1%, mean 54.52 %, median 53.2%, and standard deviation was 7.26, consequently with low permeability coefficients. Salinity is considered to be one of the principal degradation risk problems in this kind of soils [9], according to the F A 0 method for land degradation evaluation due to salt excess, which considers that some textura1 and structural characteristics of the dominant soil affect the drainage capacity of the land. This method assigns a value of 0.1 for coarse texture (sandy soils), 1.0 for moderate texture, and 1.5 for fine texture (clayey) soils. Fine texture has been identified as one of the soil characteristics that increase the risk of sodification of agricultural soils, mainly described as Vertisols and Regosols [6, 91. In the study area, we-confirmed that the dominant Vertisols are characterized by a content of 30 percent or more by mass in clayey particles in al1 horizons (layers) of the upper half-meter of the soil profile.

SOIL CLAY CONTENT (%)

conventional standards to avoid a significant depletion on the available water for crops [7], only 7.1% of the ground water from wells had an electrical conductivity lower than 0.7 dS m-', which is a type of water without any level of restriction for agricultura1 uses. In 73.4% of the cases, water extracted from wells evidenced a slight to moderate level of restriction for common uses (0.7-3.0 dS m-'), and 19.5% of the wells had a type of water with EC higher than 3.0 dS ni', with a high level of restriction for agricultural uses. Table 2 describes the water quality parameters analyzed; in most of the cases, the average values of the studied features varied within the suggested ranges for agricultural uses [7], but for ~ a " , the obtained maximum values were higher than the suggested ranges for such productive uses. Table 3 shows the correlation matrix for al1 measured parameters, as it provides both the values of the correlation coefficients (r) and the estimated levels of significance (p). High correlation was found between EC, C1, Ca and Mg (00.7). These relationships were useful to identify the elements contribution to groundwater salinity [21]. Other elements, which evidenced moderate correlations (0.5