Evaluation of Soil Moisture Changes Using the Balance Model in Zagros Oak Drying Forests (Case study: Melah Siah Forest - Ilam)

Recognition of the available moisture content and its changes in the habitat can be a dynamic ecological indicator. The aim of this study was evaluating the soil moisture storage volume with the distribution model of the Turc in the oak forest located in the Meleh-Siah Ilam.

The research was conducted from October 2013 to September 2015. Monthly rainfall and temperature data, monthly solar radiation, water holding capacity map and DEM of the region were used as input data.  To validate the soil moisture data from the water balance model, 33 humidity sensors 110 cm were installed in different locations of the habitat. Then, soil moisture was measured with TDR moisture meter in monthly time step. To evaluate the model, the mean absolute error (MAE) and mean error deviation (MBE) statistics were used and the results were analyzed.

The results showed that the accuracy of the model in estimating soil moisture in wet months is low and is overestimated, and Suitable in a dry month. Other results showed that in the first and second year of the study, precipitation was 680 mm and 383 mm, respectively and the share of evapotranspiration from precipitation is 56% and 77%, respectively. Also, in the study site the available water capacity of the soil is 125 mm, which the maximum in March and the downtrend continued until September and reached a minimum of zero. Other results indicate that the vegetation in the study area was drought tolerant in the first year (normal in terms of rainfall), in two months of August and September and increased in the second year (drought), in the months of July, August, September and October. The results also showed that 73% of the dried oak stands correspond to the areas that Soil moisture reached below the wilting point (15%) in September.

Therefore, reducing soil moisture in droughts has played an important role in the drying of oak bases and effect of the intensity and persistence of drought periods on the drying of oak trees needs further investigation.