برآورد ارزش اقتصادی آب کشاورزی با استفاده از مدل توابع تولید و روش گاردنر (مطالعه موردی ناحیه شمال خوزستان (

Considering the role of water in agricultural activities in low-water plain of the country, appropriate use of water is the best choice to have constant agricultural activities in the future. Generally, in logical pricing of water, total cost of water and purchasing power of group of consumers should be considered because a correct and proper pricing shows the consumers the exact cost of water and the way it should be consumed. As the water supplies are limited in most parts of the country (especially the area which is under study), the important role of water pricing in agriculture and its expansion seems to be necessary. 
Irrigation network in agriculture section is one of the most important irrigation networks of the Khouzestan Province. The company of utilizing irrigation networks in Khouzestan was established in 1991 to utilize a huge irrigation network. It started working in 1993. One of the purposes of this company is improvement in the structure of manpower and economizing the activities. On one hand performing the irrigation and drainage plans in different regions of Karkhe and Shavour, which the most of their members are local and poor farmers, does not only have economical profits, but also includes economical advantages such as job creation, prevention from immigration to other professions, per-capita income increase and finally improvement in local people’s living standards. After pricing and determining economical value of agricultural water in the irrigation network of North Khouzestan, an appropriate economical management of water supply can be planned efficiently to prepare a background for saving this important resource and to prevent from wasting and polluting it.
The most important role of water price can be its proper distribution among applicants based on different utilizations. In fact, one of the best policies and methods to protect water supplies is making appropriate pricing policy in different parts in order to apply an optimum model in water consumption. As a result, looking at water as an economical material, it should have proper pricing, just like other materials.
This study calculates the total cost of agricultural water based on the cost of funding, utilization and maintaining installations. In addition, it determines the economical value of  consumed water in production of main crops such as wheat, corn and cucurbits, in 2017 in Dezful, via production function, determines agricultural water’s production elasticity using results of estimated demand function in each group and region, and determines water price using Gardner method.
Water production elasticity shows farmers’ sensitivity to the water’s price changes. Any change in the form of production function that influences the calculated parameters also affects the calculated economical value. Due to the importance of proper form of function, after estimating various functions, the best function form was identified using tests and econometric criteria. 
Moreover, to compute provision cost of 1 m³ agricultural water the following relation is used,
In this study, the upper limit of water price is the shadow price or value of final output of water and the lower limit of water price is utilization price and preservation of water installations in different regions. In this path, the final cost is combination of water final output for agricultural products’ production and costs of water preservation behind the dam and other costs which are related to the organization. To make a connection between two sections (final output of water, and preservation and transformation costs) the Gardner method was used.
In order to show the effect and importance of choosing models on the amount of economical value of water in production of wheat, corn and cucurbits, five types of production function were selected, including Cobb-Douglas, generalized quadratic Translog, Transcendental, and generalized Leo Leaf to explain the relation between the production elements and the amount of crops and then, they were computed using collected data statistics.
The results of the surveys in the Transcendental function for wheat production, economical value of water and water production elasticity were calculated as 1957/33 Rials and 0/1263, respectively. And the generalized Leo Leaf function for corn production’s economical value of water and water production elasticity were calculated 2128/05 Rials and 0/4777, respectively.
In the transcendental function for cucurbit production, economic value of water was calculated to be 1485/14 Rials and water production elasticity was equal to 0/3533. The guarantied price of wheat and corn, and the average price of cucurbits in 2017 were reported to be 14300, 11502 and 7003 Rials, respectively. At the end, the shadow price of water or in other words, the economical value of water for wheat, corn and cucurbit production was determined as 1957/33, 2128/05 and 1485/14 Rials for each cubic meter water, respectively. While paid prices by farmers of each product (wheat, corn and cucurbits) are 142/1, 201/3 and 457 Rials, respectively.
In the method of calculating the entire price of agricultural water (considering funding costs, the cost of dams utilization, maintaining installations and amortization, without considering the fact that what products the farms are producing) the provision cost of 1 m³ agricultural water was calculated to be 1026 Rials, which is the minimum price of water. If 20% gross profit is expected compared with the final price, water price (based on costs of water organization of Khuzestan Province) will be 1231 Rials.
In the Gardner method, if we consider equal water payments for farmers and the organization (that is X= 50), the final prices of water for each product, wheat, corn and cucurbits, are 1594/2, 1697/5, 1358/07 Rials, respectively. The study results show that the paid price by farmers has a great difference with the real water cost in production process of the main crops of the region. Therefore, water’s low price is one of the factors causing nonoptimized consumption of water in the studied area.
Price increase of this input can lead to its consumption reduction for producing products. As a result function of product reduces per unit area. In addition, farmers may invest on new irrigation techniques that enhance the irrigation efficiency.  Following the low price of water, traditional irrigation systems are still being used and a huge amount of water is wasting. Therefore, it is recommended for water price to be adjusted based on its economical value. In addition, conditions for water proper utilization should be prepared so that agriculture production systems can improve and stabilize. This policy in a short term might dissatisfy farmers and has negative effects on their production, but as a long term policy it can result in consumption of water saving.