Introduction

Lack of water is nowadays one of the main factors for the emergence of conflicts, poverty and migration in some regions of the world (^{FAO, 2003}). This in turn responds to an environmental and social cause that many countries, particularly the underdeveloped, have not considered within their growth plans, to protect and maintain the ecosystems that provide hydrological services, except Costa Rica, but its openness to foreign investment, has brought costs in environmental terms.(^{Alier et al., 1998}). Ecosystems in general, fresh water and its natural watersheds exceed limits, political and administrative boundaries. When analyzing water management and land planning, the study framework can not be only legal, economic or even ecological; it must be sustainable, highly political and strategic, since the interactions of ecosystem services occur at different spatial and temporal scales (MEA).

Ecosystem services have had a constant conceptual evolution since the Millennium Ecosystem Assessment (^{MEA, 2005}) and are defined as the direct or indirect benefits that people get from ecosystems such as: water and food provision, regulation and control of floods, control of soil degradation and diseases, support services, soil formation, nutrient cycling and cultural services or non-material benefits. In México, ^{Balvanera et al. (2009)} carried out a comprehensive research on the state and trend of ecosystem services in our country. Without detriment to the rest of the natural capital, one of the most important ecosystem services is the hydrological one, since it is considered the base for the rest of the system services.

The importance of hydrological ecosystem services is revealed by the benefits in modifying each of its attributes.The uses of water for human, agricultural and industrial consumption are regulated by markets where the price of water only represents the costs of the energy required for its extraction, the amortization of the water infrastructure, as well as the operation and distribution costs (^{Saldivar, 2007}; ^{Avilés et al., 2009}). Unsustainable use has resulted in depletion and water pollution, because it does not consider the long-term value of the resource or the values of existence and legacy; that is, the non-use value. Efficiency in water use is a necessary but not sufficient condition for the sustainability of life itself.

Water assessment methods and techniques for agricultural use should undoubtedly contribute to strengthening decision support on the challenges and questions associated with sustainability. However, the valuation of ecosystem services involves at least two difficulties: firstly, identification and aggregation of preferences of different individuals (^{Daily et al., 2000}) and also, the uncertainty of ecosystem dynamics which turns out to be complex and multicausal (^{Carpenter and Folke, 2006}).

This economic valuation research is limited only to the irrigation water used in strawberry cultivation in the Duero River basin, Michoacán. For this purpose, the productivity valuation approach was proposed as an instrument to obtain the economic value of water. It is important to clarify that although there are aspects of water value that are not necessarily expressed in economic terms, in this paper were not considered since there is no information and these will be proposed for future analysis. The quantification of the effect of irrigation water on strawberry crop productivity was carried out for a Spring-Summer 2008 crop cycle based on three current scenarios: a) with quality water; b) without quality water; and c) without irrigation water (rain-fed agriculture).

In the first one, strawberry cultivation (main export crop) was analyzed according to the average yield in the area. In the second, the cultivation of basic grains (maize or sorghum in the Spring-Summer cycle and wheat in Autumn-Winter), and in accordance with national phytosanitary standards and international market standards for the cultivation of vegetables and strawberries. While int the latter only the maize crop under rain-fed conditions was considered.

Due to the above, this paper is basically based on the application of the concepts of ecosystem goods and services and the economic valuation. The first refers to the benefits obtained by productive areas or human settlements, from the ecological functions of natural areas located in the sub-basin. The second concept is a tool to verify, quantify and apply conservation alternatives with a sustainable social development approach; that is, the allocation of quantitative values to the ecosystem goods and services provided by natural ecosystems, regardless of whether market prices exist or not. The objective was to assess irrigation water as an input in strawberry production and to quantify the change in crop yield based on the effect of water availability and quality in the area.

Materials and methods

Study Area

The Duero River basin is located in the northwest of Michoacán, in the transition zone of the Tarasca Plateau and the Mesa Central area, delimited by the geographic coordinates 19º 40՚ 32” and 20º 20՚ 42” north latitude, and 101º 52՚ 54” and 102º 40՚ 30” west longitude, at an average altitude of 2 000 m.

The strawberry cultivation in the basin requires high layers of water, especially in the traditional system. Just to irrigate 1 500 ha of strawberry in the traditional system, which represent 8% of the irrigated area in Zamora-Jacona Valley requires 60 hm³ or 27.5% of total water allocated to the entire Irrigation District 061. The water requirements for the studied crops are: strawberry 40 000 m³, maize 6 599 m³, wheat 4 489 m³.

Strawberry production in the Duero River Sub-basin is mainly carried out in five municipalities: Zamora, Jacona and Tangancicuaro, with a total area of: 1 869 ha, with a total production of 60 491 hectares, for an average yield of 29.52 t ha^-1, strawberry growing demands quality resources so, it must meet the following official Mexican standards: NOM-001-ECOL-1996, NOM-CCA-O33-ECOL/1993, NMX-FF-006. Which regulate the presence of contaminants in irrigation water and non-industrialized food products for human use. As for the structure of costs and production systems, there are three typologies of strawberry production in the region, mainly depending on the socioeconomic strata of the farmers, as well as access to financing, in the Table 1 shows the costs structure for each production system.

Table 1 Cost structure systems strawberry production ($ ha^-1). 

Fuente: ^{Pimentel et al. (2008)}.

Economic valuation

Recognizing the three dimensions of sustainability: environmental, social and economic, monetary expression is objective and tangible, as commensurable alternative of each dimension; however, it must also be recognized that economic valuation per se, is not the entire solution of environmental problems, since it does not necessarily objectively reflect the value of these resources in economic terms, since as previously stated the value of environmental goods and services encompasses more than one dimension and not all are expressible in money (IUCN, 2001).

Therefore, decision-makers should consider the management of natural resources. For environmental management the field of sustainable application is that of the ecological economy, which is a disciplines articulator, its method and language of nature assessment, is multi criteria, monetary, ecological and ethical, distributive conflicts are incorporated and its theoretical foundations range from neoclassical economics, Marxism, ecology and ethics (Martínez, 2008). The commonly accepted framework is the theory of total economic value (VET) developed by ^{Pearce and Turner (1990)}; ^{Pearce (1993)}. This theory has the goodness to adapt the economy to the quantification of natural and environmental resources (Figure 1).

Figure 1 Total economic value (VET). Elaboration from ^{Pearce et al. (1990)}; ^{Pearce (1993)}. 

The application possibilities of valuation methods are wide as there are a number of methods for each particular good. The following is a brief description of most used methods, without aiming exhaustiveness, only the punctual differentiation of each of them is made. In order to integrate the VET, it will have to be considered the application of several valuation methods, depending on the objective to be achieved, for which, as noted above, some of them are stated.

Methods of economic valuation

Results

Conventional methodology normally considers comparative analysis of irrigated crops versus rain-fed crops. However, for this particular case a variant has been proposed in the process. First, strawberry cultivation is considered with technical premises, normative ones and of the market of the product, since this crop demands quality water and great volumes of the vital liquid and in the absence of these two conditions the cultivation of strawberry is not allowed, leaving the water production function fully defined as mentioned by (^{Young, 1996}), in which case the producer only has the alternative of growing maize and wheat, crops that are not technically and normatively demanding with water in both quality and quantity.

Consequently, the reduction of water availability in quantity and quality of the Zamora aquifer for agriculture, and particularly for strawberry producers, can face an economic loss which is reduced by 17% of the current income in strawberry cultivation. Given this situation, the scenario of lack of water resource in quality and quantity means to obtain annual net income of $ 128 200.00 against $ 21 984.70 ($ 13 081.90 + $ 8 902.80) for maize and wheat crops, a difference derived from the loss of ecosystemm service (Table 2).

Table 2 Comparative analysis of irrigated Strawberry vs irrigated maize and wheat. Comparison of unit production costs and net profits of irrigated agricultura, maize (traditional technology). 

Fuente: elaboración con base a los datos oficiales del ciclo agrícola 2007 (ASERCA, 2007).

In an extreme case, when it is no longer feasible to plant vegetables due to scarce water resources and because there is no quality in it, this scenario is exemplified below (Table 3).

Table 3 Comparative analysis of irrigated maize and wheat vs rain-fed maize. Comparison of unit production costs and net profits of irrigation agriculture, maize (traditional crop) and mechanized rain-fed maize. 

Fuente: elaboración con base en los datos oficiales del ciclo agrícola 2007 (ASERCA, 2007).

Discussion

The use of the production function technique is applicable when water contributes in a significant fraction of the product’s value, as in this case because if an input is omitted or underestimated in the function, its value will go to the residual value (water), overestimating this value, in this studied case there was a reliable prices statistics and products that intervene in the production cost of the analyzed crops.

The production function covered the premises of the method, all important inputs were identified, in particular the water function and productivity level was established in quality and quantity for the production of strawberry crops and its total dependence to water resource. There is also no government intervention on products or inputs, so that these assumptions are satisfactorily fulfilled.

The application of the production function methodology can be problematic (^{Aylward and Barbier, 1992}), when there is more than one production function. In this study, the analysis was carried on a crop, although the water is used for a wide range of crops, only strawberry crop was considered, because only 2 000 ha consume about 30% of the total water available of the Duero River aquifer.

The results of this analysis make it evident that having quality water and sufficient quantity for the cultivation of strawberries, allows to obtain a differential of $106 215.30 ha^-1 compared to the cultivation of maize and wheat. This differential is largely due to the fact that the international market allocates to quality water, which comes from the Zamoranos aquifers (^{Peniche, 2007}), the results confirm that in the basin of the Douro River, quality water is simply lacking in price and is not found in the costs scheme of strawberry production; from the results of the above analysis, a value of $3.67 per m³ was assigned to the water^..The estimated values are representative of the current value of water in agriculture to the extent that strawberry and maize and wheat are the agricultural products occupying the largest area in terms of vegetables and basic grains in the area and, presumably, strawberry cultivation is the one that consumes more water.

Moreover, these results show that the difference between the cultivation of strawberries and maize, water and its quality is decisive for strawberries cultivation, and a productivity level of 29.52 t ha^-1 is obtained, a net inflow of $112 047.00 ha^-1 vs maize crop. Moreover, there is a water value of $3.67 m^-3 in the strawberry crop, instead from irrigated maize to rain-fed maize, it is drastically reduced to $0.88 m^-3. However, if the price of $3.67 m^-3 is compared to $1 315.79 m^-3, which is the cost of bottled water in the area for human consumption, there is simply no proportion whatsoever, then this analysis shows the importance so far not recognized of the ecosystem service that serves a major consumer such as the basin agriculture.

Conclusions

The application of the productivity change method in the evaluation of the ecosystem service of water supply as an input in the food production, in the strawberries cultivation in the Duero River basin, Michoacán, allowed to identify the cause-effect relation; the production of berries and vegetables in the Duero River basin, in particular strawberry cultivation depends on the quality and quantity of water, resulting in a net income of $128 200.00, on the other hand, if this ecosystem service is lost, net income is reduced to the production of maize and wheat and for these crops the net income is only $21 984.70 which means to stop receiving $106 215.00, on the other hand, if that level of unsustainable irrigation water use-consumption continues in the short term will affect the welfare of society; the search for profitability shows the environmental impact of strawberry cultivation on water resources.

The economic valuation is particularly important because it allows to see a little known face of production systems and is a determinant indicator to analyze the effects of the production/consumption relationship of ecosystem goods and services provided by the ecosystems in the Duero River basin. By translating the physical effects into monetary values, it allows the comparison between different environmental management alternatives to choose the one that best suits the welfare of society; that is to say, the one that has the least monetary value for its externalities and, consequently, shows the least damages to ecosystems.

The quantity and quality of the irrigation water in the study area shows that this ecosystem service represents 83% of the net income of the producer, against 17%, if the service is lost, changing from producing an export product like strawberry to only planting maize and wheat in irrigation conditions in its two spring-summer and autumn-winter production cycles.

Against the problems addressed in this paper, elements are proposed that start from the interest in basic valuation methods to place the issue of water resource management in the discussion table of valuation in México. The importance of the water resource is not under discussion, although water as a human right does not mean giving it away, but a minimum level of undeniable provision must be established above which users must make responsible use of it and generate culture for the payment of the ecosystem service and not only of its administration.

Therefore, it is the responsibility of all social actors who benefit from the water resources of the basin to contribute or, if necessary, support studies that allow the knowledge of the capacity of generation and provision of ecosystem services at the level of biophysical environmental units (UAB) and their anthropic interrelationships in the Duero River basin, since the pressure exerted on the aquifer by the need for water in quantity and quality may in the medium term; Generate shortage and depletion of it. Social responsibility must be the constant in any economic activity; it must contribute to the conservation and maintenance of natural ecosystems that provide ecosystem goods and services such as water.