Introduction
Environmental flow requirement (EFR) is defined as the flow that is necessary to ensure the existence of habitats in water resources systems. Hydrological methods are almost the most straight-forward approach in rivers. These types of methods are also known as desktop methods that rely on annual, monthly or daily flow discharge data of the river. There are two main limitations for desktop approach. First, in this type of approach, regional ecological values are not considered directly. Secondly, this type of approach has little defense capability in interactions of water allocation, but because of having some advantages such as simplicity, it is used in many countries. But it is obvious which development of localized desktop approach is so necessary and also is so useful. It can be used as a flow index for environmental flow assessment in different projects. Main contribution of present research is development of new desktop method for environmental flow assessment in southern Caspian Sea basin with focusing on regional ecological values.
Material & Methods
The Lar National Park is located 55 km at east of Tehran. Its mean elevation from open Seas is 2531 m and its area is 27000 Ha approximately. In this park, Brown Trout (one of Iranˈs most unique species) inhabits. This park is one of the original habitats of Brown Trout which has unique properties. In this park, predation of Brown Trout is forbidden and is protected by the Iranian Department of Environment (DOE). Water quality in all of the streams in the park in a good condition and there are minimum water quality issues for brown trout habitats, also brown trout is dominant aquatic so there is no effective competition in streams. Hence this park is an ideal place for development of hydrological flow index based on real life of aquatic in their habitats and physical habitat effects. Since the development of physical habitat simulation, physical habitat models became an important tool for river management .Aquatic habitat simulation models have been used for fish in water resource management, particularly in North America. The Physical Habitat Simulation is considered to be the first of these fish habitat models and is now being applied worldwide. In the present research 1-D hydraulic simulation in combination of physical habitat simulation is used to simulate physical habitat for Brown Trout. Quantified ecological modeling describes flow changes in physical components of the system and translates them into an estimate of the quality and quantity of microhabitat for aquatic organisms. The most commonly used output from these types of models is Area Weighted Suitability (AWS). This factor is computed within the reach at a specific discharge from:

(1)

Where Ai is the surface area of cell i and Ci is the combined suitability of cell i (i.e., composite of depth, velocity and channel index individual suitability). Common method in estimation of C is consideration of minimum value of depth, velocity and substrate suitability for development of combined habitat suitability in each habitat cell. Description of river condition is carried out in three main conditions which are maximum protection or outstanding, mid protection or good and minimum protection or poor. Two main habitats are selected for implementation of ecological modeling which are Elarm which is as a main habitat for fry and juvenile Brown trout and Absefid which is as a main habitat for adult Brown trout. Slope of Elarm is smaller than 2% and slope of Absefid is larger than 2%. Hence development of flow index for environmental flow is carried out based on these two types of rivers. Because popularity of mean annual flow(MAF) as an acceptable index in determination of environmental flow requirement, development of hydrological flow index was based on MAF in each river.

Results & Discussion
Habitat time series is shown in figure 1. In this figure you can see the alteration of AWS in different months

Figure 1. Habitat time series in simulated habitats( figure above is Absefid and figure below is Elarm)
According to habitat time series, environmental flow regime is assessed in two habitats in three stages. First stage was maximum protection or outstanding condition which maximum area weighted suitability is available for aquatics in river. Based on ecological negotiations, minimum acceptable AWS was 50% of maximum AWS, hence In stage 2 and 3, 75% and 50% maximum area weighted suitability is available. Estimated environmental flow regime is displayed in figure 3. Lar method recommendations for assessment of environmental flow requirement are displayed in Table 1. It should be noted that seasonal scale is considered.

Figure 2. Environmental flow regime ( figure above is Absefid and figure below is Elarm)
Table 1: Lar method recommendations in EFR assessment
River condition Spring Summer Fall Winter
2%> 2%< 2%> 2%< 2%> 2%< 2%> 2%<
Outstanding 70 130 135 425 120 300 70 135
Good 60 35 105 100 100 60 60 35
Poor 45 20 85 30 80 25 45 20

Conclusion
In the present research, a desktop method is developed based on regional ecological modeling in southern Caspian Sea basin. Because of special features of Brown trout habitats in Lar national park, This place is chosen for ecological modeling and development of hydrological flow index based on mean annual flow. According to results, minimum flow need for rivers which slope is larger than 2% are 20%,30%,25% and 20% percentage of MAF in spring. Summer, fall and winter respectively but these values in river which slope is smaller than 2% are 45%,85%,80% and 45% percentage of MAF.