Introduction

In the population census from 2010 indicated 112 336 538 inhabitants showed marginalization (INEGI, 2011). Also, a high percentage lacked of services to some extent, including availability of drinking water at home. The difficulty to supply water in sufficient quantity and quality to inhabitants of Mexico City is very large and the process very expensive, for which its inhabitants must be extremely careful in handling and conservation of water available, as scarcity of the resource can condition development and cause serious conflicts among users (^{CONAGUA, 2000}). In 2010 to provide a cubic meter of water to Mexico City cost about 7 pesos (US $ 0.53), but the people from capital paid an average of $ 2 m³ (US $ 0.15 m3).

Water scarcity is one of the problems in natural resources that threaten humanity. The center of Mexico is the region with less water, with an annual average availability per capita of 190 m³, while in the southern border count with 25 843 m³ per capita. So the problem is obviously more critical in the central states, which justify this study in the Iztapalapa delegation, being an overpopulated area.

Iztapalapa is the most Eastern delegation from Mexico City, which complicates the provision of drinking water for the area, since one of the main supplies of water for the city (the Cutzamala System) enters from the north. The water offer consists of underground water supplies found around Sierra Santa Catarina; however, these are not enough to meet the demand for water in the delegation, which is complemented considering that stated above with water obtained from the Cutzamala system, but still not enough to meet demand. During the dry season, water scarcity increases, especially in the highlands of San Lorenzo, Paraje San Juan and Santa Catarina which are the most marginalized areas from the delegation.

This research deals with water for urban use, which is a trend topic because the problems in quality and quantity of available water is increasingly acute in central and northern Mexico and particularly in the delegation Iztapalapa from DF. The overall objective was to estimate the economic value of drinking water in the Iztapalapa delegation, by the contingent valuation method (CVM). For which the following specific objectives were raised, estimate willingness to pay (DAP) to improve the quality of water service for urban use, taking into account socioeconomic variables (age, education, marital status, sex, income and shortages) and determine the perception of the current drinking water quality and desired quality. The general assumption that economic valuation of drinking water is a function of education, age, income, marital status, sex and water shortages arose. Furthermore, the perception of the quality of drinking water is bad and that the shortage affects a lot of users.

In their reserach on willingness to pay for improvements in the quality of drinking water ^{Jordan and Elnagheeb (1993)} in Georgia, United States of America, using the contingent valuation method, estimated an average of DAP of $ 5.49 dollars monthly, over the rate that were already paying, and those with access to private wells up to $ 7.38 dollars. However, ^{Polyzou et al. (2011)} found that, for different reasons, not always the users of drinking water service are willing to pay for an upgrade as they found that only 40% of respondents were willing to contribute some amount to improve the quality of drinking water. ^{Harner (1996)} in his research on the value of drinking water in partially monetized rural economies found that users of drinking water through the forms of cooperation in them is present, are willing to provide working days to get improvements in such service.

Materials and methods

The methodology consisted of applying the contingent valuation method (CVM), which begins with the design and implementation of a questionnaire to users whose sample size was determined by applying sampling techniques.

To obtain honest responses in the survey it parts from a credible situation on the conditions under which the natural resource water in Iztapalapa are, although this is hypothetical. The survey was designed so that interviewers can identify the main variables that influence the decision of respondents, avoiding bias and facilitating data recording to enable subsequent econometric modeling.

As it was worked with a hypothetical situation and subjective responses, should be taken into account certain rules and elements from the survey to ensure a good design of it. For this, the survey was prepared according to the general guidelines given by ^{Mitchell and Carson (1989)}, but the emphasis is on the survey items listed below (^{Duffield and Patterson, 1991}; ^{Cooper, 1993}).

Formats for DAP question. The second set of questions are the most important and to which more attention is paid during the design process. The sub-elements of this block are: 1) the hypothetical market; 2) payment vehicle; and 3) the question about DAP (^{Barzev, 2004}).

In the designed questionnaire the question is elaborated according to that stated above and each respondent is questioned with different monetary amounts, chosen at random and in each print of the questionnaire mark the rectangle (cell) containing the amount at random; however, only considered amounts in the payment card (^{Monroy, 2011}).

Sampling and sample size. The statistical method used to determine the sample size in the Iztapalapa delegation was simple random sampling (SRS), which is a selection method of n units in a set of N so that each _NC_n sample have the same chance of being selected. In practice, a simple random sampling is performed unit per unit (^{Cochran, 1984}).

Sample size: the target population were users older tahn 18 years who may be aware of water use. Each questionnaire was assigned a random number of monetary amount to be asked so the the interviewer could not introduce bias when performing the interview with users of drinking water. The delegation has 1.8 million inhabitants according to the Population Census and Housing from 2xxx. The sample size was 95 inhabitants.

Modeling: the linear model was applied for this research, which is used in the nLogit / Limdep program. The general logistics model is presented below:

P(SI)= α₀ + β₁X1 + β₂X2 + β₃X3 + β₄X4 + β₅X5 + β₆X6 + β₇X7 +β₈X8 + β₉X9 +εe

Where: P (SI)= is the probability of a positive response from DAP; X= is the set monetary amount; X2= age; X3= education; X4= marital status; X5= gender; X6= family size; X7= family income; X8= perceived quality; X9= perceived scarcity of water and error.

Results and discussion

Conclusions

The economic value of water was estimated by the contingent valuation method, obtaining an average willingness to pay (DAP) for the inhabitants from the delegation of $ 62.63. 62% of people would be willing top ay an extra amount for quality, quantity and perceived supply of wáter to be improved, 38% of people would not be willing to pay some extra amount due to the current payment or because do not believe in the authorities.

The 93% of respondents said that water from the delegation has some degree of contamination from moderate to very high, which is why mentioned that it was urgent to make service improvements. Regarding to economic analysis, of estimated coefficients from the logit model there is that at a higher price that was assigned to the valuation of water, the probability of getting a positive response from the respondent is lower. The lineal model estimated in nlogit/limdep program showed that only the price variable is significant in statistical terms in the model to the desired reliability, and the others close to be significant are education, sex and age.