Determining homogeneous areas of water storage capacity using concepts of algebraic maps

Introduction
Available water holding capacity (AWHC) is a key property for quantifying the amount of water available to plants. This c property determines the amount of water required for the crop and the irrigation time interval and is related to the inherent productivity of the soil. Determining the homogeneous zones of available water-holding capacity management is a possible way to evaluate the contribution of the soil in the optimal input of irrigation water in paddy fields. Because with its help, it is possible to manage the appropriate time of drying and wetting of paddy fields in intermittent irrigation. The rice fields of Guilan province, located in the north of Iran, with an area of about 238,000 hectares, are one of the most important rice cultivation areas because more than 30% of the country's rice is produced in this area. Irrigation in this area is done in the form of uniform flooding for easier transfer of rice seedlings, better water retention and weed control. In recent years, due to drought, population growth and increasing urban and industrial demand for water and inefficient management of flood irrigation, an attempt has been made to manage irrigation in the form of intermittent irrigation. Therefore, determining the homogeneous zones of water storage capacity management is a possible way to evaluate the contribution of soil in the optimal input of irrigation water in paddy fields. Because with its help, it is possible to manage the appropriate time of drying and wetting of paddy fields in intermittent irrigation. Water resource management is a priority to reduce productivity instability and negative socio-economic effects. For this purpose, homogeneous water storage capacity management zones were determined in Kouchesfahan region to investigate the physical-hydraulic conditions of paddy soils, dividing the entire region into zones with the same potential for water storage capacity and investigating its relationship with rice yield.
Methodology
A total of 131 undisturbed and undisturbed soil samples with uniform geographic distribution were prepared from the rice root growth area and some chemical, fertility and physical-hydraulic properties were measured. Rice yield was also measured in most of the soil sampling points. Then, among 14 characteristics, seven characteristics (clay, organic carbon, linear expansion coefficient, saturated hydraulic conductivity, average particle diameter, accessible water and integral energy) that can affect the water holding capacity in the soil were selected. The number of homogeneous zones of water storage capacity was determined using two statistics, fuzzy efficiency index and modified classification entropy index. Then, by using the concepts related to algebraic maps, the integration of information layers was done in the GIS environment and the relevant maps were prepared.
Result and discussion
The results of the fuzzy efficiency index and the entropy index showed that the investigated area can be divided into four water storage capacity areas. The lowest and highest value of the average yield was seen in the first and fourth zones, respectively. A significant difference was seen between available water, organic carbon, COLE, integral energy and Saturated Hydraulic Conductivity (Ks)
in four management zones, but no significant difference was seen in the amount of clay and the mean diameter of the pores. In the first zone, organic carbon, Coefficient of linear extensibility (COLE), Mean of Soil Pore Size Distribution (dmean) and Ks showed the lowest values, but in the fourth zone, all these properties have the opposite behavior. These results were showed that the available water-holding capacity ty increases from zone one to zone four. Therefore, zones one and two will be particularly sensitive during drought and lack of moisture during the growing season, and the management of these zones needs special attention. In these zones, the irrigation cycle (irrigation time interval) should be shorter than the other two areas, in order to avoid the occurrence of drought stress. The soils of the studied area were uneven in terms of water retention. These results showed that the uniform (fixed) management of water consumption, in addition to increasing costs, can also lead to the waste of a large amount of water. In this situation, location-specific irrigation management can be more efficient in sustainable economic production.
The comparison of the estimation map of homogeneous zones shows the water storage capacity and yield, in some zones, although the soil conditions are suitable in terms of moisture conditions, the yield is not in optimal conditions. It seems that until the state of fertility is not favorable, physical conditions cannot show their effects in performance well. In other words, until the lack of fertility is not resolved, restrictions or suitable physical conditions will not have clear effects on performance.