Study area
The field studies were conducted on the side slope of a forest road hillslope in the Shirghalaye basin forest, Guilan province, Southern Caspian Sea between 49° 52´ to 37° 5´ N latitude and 49° 55´ to 50° 8´ E longitude. The study area is 2588 ha (Fig. 1). This mountainous area has an altitude ranging from 1120 to 1680 m. The major soil type is clay loam soil. The general soil thickness is 0.5–1.5 m, with a humus layer of 0.15–0.50 m and a litter of 0.10–0.50 m. The soil pH varies from 6 to 7.
The forest lands were predominantly covered by Oriental beech (Fagus Orientalis Lipsky), Alder (Alnus glutinosa & Alnus sabcordata), and Hornbeam (Carpinus betulus). The mean tree height is 21 m and stand density was measured as 180 trees per hectares. The mean annual precipitation recorded at the closest climatology station is 1200 mm. The maximum mean monthly rainfall of 120 mm usually occurs in December, while the minimum monthly rainfall of 25 mm occurs in August. The mean annual temperature is 15 C˚ with the lowest values in February. Due to widespread timber harvesting in this area, there is a large network of forestry roads (46.8 km). The study area consists of a number of hillslopes with a slope between 17.3–51.9% and constitutes a limited area of about 4 km2.
Runoff Measurement Using Field Plots (Rfp)
In this part of the study, 45 plots of runoff with 2 m2 dimensions were established on the forest road hillslopes. The material of the plot walls was waterproof and made of aluminum.
Various embankment slopes and crop cover percentages were used (Fig. 2). The plots were established on the road hillslopes. The penetration depth of the plot walls in the soil is 10 cm. One runoff plot was established per kilometer of the road.
The coverage rate was determined by taking a digital picture at the location of each plot. Slope percentages were measured using an inclinometer.
The soil samples from the depth interval 0-10 cm were collected with a soil hammer and rings (diameter 5 cm, length 10 cm) outside of the plot (Fig. 2), put in polyethylene bags, labeled, and brought back to the laboratory where they were promptly weighed. Soil samples were oven-dried at 105° C for 24 h. The soil moisture content in the samples was measured gravimetrically after drying (Kalra and Maynard 1991). Soil texture (clay, silt, and sand percentages) was determined in the laboratory. Particle size distribution was determined by using the Bouyoucos hydrometer method (Kalra and Maynard 1991),
Soil bulk density was calculated as Equation (1):
Db = \(\frac{Wd}{VC}\) (1)
where Db is the dry bulk density (g cm−3), Wd is the weight of the dry soil (g), and VC is the volume of the soil cores (196.25 cm3).
Initial soil moisture was calculated as Equation (1) (Kalra and Maynard 1991):
Ms= [(Ww – Wd)/Wd]×100 (1)
Where Ms is the soil moisture content (%), Ww is the moist soil weight (g), and Wd is the dry soil weight (g). Two rainfalls of 60 and 45 millimeters were considered as the designated storm rainfalls to evaluate the runoff generation potential. The rainfall intensities were 22 and 30 millimeters per hour, respectively, the return period of rainfall in this study was 20 years. At the end of the lower slope of each plot, a runoff collection tank was installed. Runoff volume was measured after the designated rainfall (Las Heras et al. 2010). In fact, after the rains, we emptied the reservoirs and waited for a high-intensity rainfall event to be recorded. After recording the rainfall event, that event and its data were used as designated rainfall. The amount of rainfall was more than the initial loss of soil in the area and led to runoff generation. The initial loss in this study was estimated to be 5-18 mm. The initial loss was estimated based on the amount of rainfall before the start of runoff in the entire plot area (n=45).
Finally, the plots were used to measure runoff on the selected hillslopes.
Prediction Of Rgp Using Anns
We used a multi-layer perceptron (MLP) network in NeuroSolutions software to predict the runoff generation potential on the hillslopes. ANNs is used by an enveloping the optimization algorithm to adjust the weight of each neuron, completing the learning process for that case. A multilayer perceptron is a class of feed forward artificial neural network. An MLP consists of at least three layers of nodes. Except for the input nodes, each node is a neuron that uses a nonlinear activation function. Learning was performed in the perceptron by changing connection weights based on the error values in the comparison with the experimental values. We used a feed-forward neural network to predict the RGP volume. The factors slope gradient, vegetation canopy percent, soil moisture, clay, silt, and sand percentages (soil texture) were selected as inputs and the runoff volume as the output. Finally, RGP were evaluated for 45 plots in two rainfall events (90 samples of rainfall run off). After normalizing all data, they were then split into training/test data sets: training data (70% of all data) and testing data (30% of all data). In terms of optimizing the network structure, it was determined that the optimal transfer function, optimal inputs or the real affecting factors, the determination of optimal learning techniques, and the appropriate number of neurons. (Isik et al. 2013). After training and network optimization, the network was established. Finally, an optimized and tested MLP network was created for the reliable prediction of quantitative amounts of RGP. To determine the optimal structure of the network (optimization inputs, the optimal transfer function, and optimal learning technique) the trial-error method was used in the NeuroSolutions medium. After each trial-error, network assessment was performed by evaluating the error values and the correlation coefficient between the predicted values and the observed data. Finally, after achieving the optimal network structure, the network testing was performed.
An MLP with tangent hyperbolic transfer function, LM back-propagation technique, two hidden neurons, 1000 epoch, and the optimal inputs was selected as an optimal network to predict RGP. To evaluate the performance of the ANN model, statistical criteria was used such as the coefficient of determination (R2), mean square error (MSE) and mean absolute error (MAE). The coefficient of determination is always between zero and one and how much it is closer to one, to indicate the better performance of the model (eq. 1):
MSE is based on the difference between actual and predicted values which are evaluated and the value which is lower and closer to zero indicates the better performance of the model (eq.2):
\(MSE=\frac{\sum \left({Q}_{i}-{\widehat{Q}}_{i}\right)}{n}\)
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(2)
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MAE value changes from zero to infinity. How much it is closer to zero indicates the better performance of model (Khaleghi et al. 2014) (eq.3):
\(MAE=\frac{1}{N}\sum\limits_{{i=1}}^{N} {\left| {E{}_{i}} \right.}\)
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(3)
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Where Qi is the observed value, is the simulated value and is the mean of the observed data and is the mean of the simulated data and n is the number of data. Ei or absolute error is the difference between the predicted and observed values. Further, a sensitivity analysis of the model inputs was performed in order to determine key parameters to RGP in forest road hillslopes. This analysis is performed after the network testing. In other words, this analysis is used to assess the affecting factors in the network output or RFP values. The ANN performance was evaluated using error values (MSE, MAE, and R2) and the comparison between the predicted and observed values.
Mapping Of Rgp By Coupling Anns And Gis
One of the goals of our study was to couple an ANNs and GIS to predict RGP for the forest road hillslopes. In other words, we can predict and map RGP for runoff values by coupling ANNs and GIS on the forest road network.
At this stage, the geo-referenced layers of the network inputs (tested network) in GIS with raster format were combined. Pixel size was defined one by one meter based on field plot size and data. Finally, geographical coordinates and quantitative amounts of affecting factors in RGP were acquired for each cell. These data were transferred from the GIS to Excel medium. The output data serves as the input of the tested MLP network to predict runoff volume in the study area.
Next, the predicted RGPs were transferred from the ANNs back to GIS. An RGP map was generated using the predicted runoff values and GIS capabilities. The RGP map was prepared by classifying the map in GIS. An evaluation of ANNs performance in RGP mapping was performed through the test stage. Further, the performance of coupling ANNs and GIS was evaluated through overlaying the observed runoff values (field plots data) on the predicted runoff values in the map.