Floods are natural disasters that bring devastation and wide spread miseries to life. Though complete immunity from floods is not possible, yet the damage potential due to floods can be reduced through various structural and non-structural measures. The structural measures are capital intensive compared to non-structural measures, which are less expensive. Non-structural measures are basically used to provide flood forecasts and warning to people who reside in different flood zones (Kumar et al., 2001).
Floods are natural phenomena and inherently complex to model. The UK Flood Estimation Handbook (FEH) notes that many flood estimation problems arise at ungauged sites for which there are no flood peak data. In such cases, the hydrologist is faced with the difficult task of estimating flood event magnitudes from basin properties and/or regional climatology. The FEH recommends that, wherever possible, such estimates should be based on the transfer or analogous data from sites that are hydrologically similar in terms of basin area, rainfall and soil type (Dawson et al., 2006).
The quantity of runoff resulting from a given rainfall event depends on a number of factors. Different factors have been used to generate the stream flows. It is possible with standard statistical regression techniques to forecast flood based on basin descriptors-for example, derived from basin area, wetness and base flow index (Dawson et al., 2006). However, Robson and Reed (1999) state that flood estimates made from basin descriptors are, in general, grossly inferior, to those made from flood peak data (Robson and Reed, 1999).
The aims of the present investigation are thus twofold: (1) to explore the potential application of a new regression model, Artificial Neural Networks (ANNs) and a hybrid model solutions to the problem of flood estimation in ungauged basins; (2) to compare ANNs model prediction skill with that of the regression and the hybrid model.
ANNs have been used to perform hydrological modelling operations for over a
decade. Since the advent of effective training algorithms for neural networks
in the mid 1980s (Rumelhart and McClelland, 1986), neural solutions have been
applied to a wide range of hydrological problems, such as rainfall-runoff modelling
and river discharge (or stage) forecasting (for a review of forecasting applications
(see Abrahart et al., 2004; Govindaraju, 2000). There have, however,
been relatively few studies involving the application of ANNs to flood estimation
at ungauged sites. For example, at the regional scale, Liong et al. (1994)
investigated flood quantile prediction for ungauged basins in Quebec and Ontario
(Liong et al., 1994). Muttiah et al. (1997), investigated 2-year
peak storm discharge predictions for river basins in the United States (Muttiah
et al., 1997). Hall and Minns (1998) related the scale and location parameters
of the Extreme Value Type 1 (EV1 or Gumbel) distribution for annual floods to
six basin characteristics in two flood regions of the UK in subsequent experiments
(Hall and Minns, 1998). Hall et al. (2000) used between four and twelve
input basin characteristics to predict the same two EV1 parameter outputs using
data from sites in Sumatra and Java (Hall et al., 2000) whereas Dastorani
and Wright (2001) found that seven basin inputs were sufficient to predict the
index flood for selected basins in the UK (Dastorani and Wright, 2001). This
study discusses the application of a hybrid model consisted of ANNs and a new
regression model to estimate 5, 10, 15, 20, 25, 50, 100, 200, 500 and 1000 year
flood event magnitudes using eleven input variables at such sites.
MATERIALS AND METHODS
Study region and data: The Urmia Lake basin is located in the North-west of Iran. This basin lies between 35° 40' and 38° 29' N and between 44° 13' and 47° 53' E and has a drainage area of 52700 km2. The climate of the basin is continental semi-arid. The average annual precipitation and the average annual stream flow are about 398 mm and 4.5x109 m3, respectively (Hassanpour, 2007). In this research, we applied MLP network, Elman network and a new nonlinear regression model to forecast T-year flood events in western region of Urmia Lake in 2006. The study region is consisted of six sub-basins. Physiographical and climatic data and information were obtained from West Ajarbayjan Water Authority. These data included basin drainage area, basin perimeter, Longest drainage path, basin mean slope, river mean slope, gravelius factor, form factor, time of concentration, 2-year rainfall and mean altitude of basin above sea level.
Nonlinear Regression Model (NRM): In the present study, a new regression model which does not have been applied within the field of regional flood modeling, is defined as follows:
where, QTr is T-year flood event magnitude (m3/s), Tr is the return period (year), α and β are the parameters which are defined based on the physiographical and climatic data as follows:
where, A is the basin drainage area (km2), P is the basin perimeter (km), Lr is the Longest drainage path (km), Sb is the basin mean slope (%), Sr is the river mean slope (%), Fg is the gravelius factor, Fb is the form factor, Tc is the time of concentration (hr), R2 is the 2-year rainfall (mm), H is the Mean altitude of basin above sea level (m) and ai and bi are the model constant coefficients.
Multi-Layer Perceptron Network (MLP): Although there are now a significant number of network types and training algorithms, this research will focus on the Multi-Layer Perceptron (MLP) and Elman networks. Figure 1 and 2 provide an overview of the structure of these networks, respectively.
In this case, the ANN has three layers of neurons (nodes) -an input layer,
a hidden layer and an output layer. Each neuron has a number of inputs (from
outside the network or the previous layer) and a number of outputs (leading
to the subsequent layer or out of the network).
||Multi-Layer Perceptron (MLP) network
A neuron computes its output response based on the weighted sum of all its
inputs according to an activation function (in this case the tangent sigmoid).
Data flows in one direction through this kind of network-starting from external
inputs into the first layer (the predictors), that are transmitted through the
hidden layer and then passed to the output layer from which the external outputs
(predictands) are obtained. The network is trained by adjusting the weights
that connect the neurons using a procedure called error back propagation. In
this procedure, the network is presented with a series of training examples
(predictors and their associated predictands) and the internal weights are adjusted
in an attempt to model the predictor/predictand relationship. This procedure
must be repeated many times before the network begins to model the relationship
(Dawson et al., 2006).
Elman network: Elman Networks are a form of recurrent Neural Networks
which have connections from their hidden layer back to a special copy layer.
This means that the function learnt by the network can be based on the current
inputs plus a record of the previous state(s) and outputs of the network. In
other words, the Elman net is a finite state machine that learns what state
to remember (i.e., what is relevant). The special copy layer is treated as just
another set of inputs and so standard back-propagation learning techniques can
be used (something which isn't generally possible with recurrent networks).
At each time step, a copy of the hidden layer units is made to a copy layer.
Processing is done as follows:
||Copy inputs for time t to the input units
||Compute hidden unit activations using net input from input units and from
||Compute output unit activations as usual
||Copy new hidden unit activations to copy layer
RESULTS AND DISCUSSION
Nonlinear regression model: The parameters of the nonlinear model (α
and β), are calculated using Excel software and the Linest function. In
order to determine the best variables that have the most effect on the good
performance of the multiple regression model, T-year flood magnitudes are estimated
using each variable. The first variable is chosen based on the maximum correlation
coefficient between observed and estimated discharges. The second effective
variable is also chosen by testing the effect of both the rest variables and
the selected variable on the model and etc.
||The best variables chosen by the nonlinear regression model
This will be continued until there is no difference between two successive
maximum correlation coefficients or the last obtained correlation coefficient
is enough high (Table 1). As can be seen from Table
1, the correlation coefficients related to No. 5 and 6, are almost the same
and the one of No. 6 is excellent, so, there is no need to add the rest variables
to the input variables and it is better to remove them. According to Table
1, the more the input variables, the better the obtained results. As can
be seen from No. 4, the presence of Sb and Sr in the model
is not reasoned because of their similar influences on T-year flood magnitudes.
Also, this model shows good and almost similar results when it uses more than
(or equal to) four input numbers. So, the data of Fb, A, Sr
and H are recommended as the best variables for flood estimation in west basins.
In the next two sections, we examine that MLP and Elman network whether confirm
these 4 variables as the best ones or not.
MLP network: The selected data from the regression model are considered as the neural networks inputs. They were normalized in the range of [-1, 1] because of using the tangent sigmoid transfer function in the hidden layer, as follows:
where, Pi is the inputs, Pmin and Pmax are
minimum and maximum input respectively and Pn is the normalized inputs.
In order to improve generalization, all data were divided into three sets: training
set (75% of data), validation set (30% of training set) and test set (25% of
data). Twelve training algorithms such as: Basic gradient descent (gd), Gradient
descent with momentum (gdm), Adaptive learning rate (gda, gdx), Resilient back
propagation (rp), Fletcher-Reeves conjugate gradient algorithm (cgf), Polak-Ribiére
conjugate gradient algorithm (cgp), Powell-Beale conjugate gradient algorithm
(cgb), Scaled conjugate gradient algorithm (scg), BFGS quasi-Newton method (bfg),
One step secant method (oss) and Levenberg-Marquardt algorithm (lm) were applied
to train the network. In this research, we left most of the training parameters
of these algorithms at the default values, except the performance goal and maximum
number of epochs to train that we modified their default values and sat them
equal to 0.008 and 120, respectively. After training, network was tested based
on the test data and a linear regression was performed between the network outputs
and the corresponding targets by putting the entire data set through the network
(training, validation and test) to measure performance of the trained network.
It returns to three parameters. The first two, m and b, correspond to the slope
and the y-intercept of the best linear regression relating targets to network
outputs. If we have a perfect fit (outputs exactly equal to targets), the slope
would be 1 and the y-intercept would be 0. The third variable returned by regression
analysis is the correlation coefficient (r-value) between the outputs and targets.
It is a measure of how well the variation in the output is explained by the
targets. If this number is equal to 1, then there is a perfect correlation between
targets and outputs. Finally, the optimum number of the hidden neurons for each
algorithm and input numbers was determined based on the maximum correlation
coefficient value (Table 2). It can be seen that all algorithms
dont show good results with one input variable (No. 1), in other words,
the form factor (and return period) variable is not sufficient for precisely
flood forecasting. Gd and gdm algorithms were recognized as weak algorithms,
because of their bad results compared with other algorithms and the regression
model. Rp and cgp have shown the best results compared with other algorithms
of back propagation and conjugate gradient algorithms, respectively. bfg algorithm
is the best algorithm with average correlation coefficient of 0.916. Maximum
average correlation coefficient of all algorithms with 1 to 6 input numbers
was equal to 0.933 related to the network with four input variables (No. 4).
So, it can be concluded that MLP network is able to estimate flood using just
four variables and there is no need for more inputs variables. In other words,
MLP network is able to recognize the effective variables needed for flood forecasting.
More input variables do not always cause to a good performance of MLP network.
Elman network: Elman network was trained as similar as MLP, but only
gdx algorithm was applied to its training. Table 2, also shows
the results of this network. It can be seen that, this network with six input
variables (No.6) has the best performance because of the maximum correlation
coefficient value (0.906) and there is an almost remarkable difference between
it and the one of No. 4 (0.888).
||The results of MLP and Elman networks
|1gdx is relevant to Elman network
||Comparison of the observed and the estimated flow of the regression
model (No. 4)
||Comparison of the observed and the estimated discharge of
bfg algorithm (No. 4)
But, its result in No. 5 that the Sb factor is added to the inputs,
is worse than No. 4. So, Elman network could recognize the best variables for
flood forecasting. According to the Table 2, it can be seen
that Elman network has a low convergence speed. Also, this network needs more
hidden neurons (10) than MLP (8). Also, gdx algorithm of MLP network shows better
results than the one of Elman network.
Because of too many graphs obtained from these models, Fig. 3-5 compare the observed flow with the estimated flow of the nonlinear regression model, the best algorithm of MLP (bfg) and Elman network using only four variables (No. 4).
In order to determine the best variables for flood forecasting in west basin easily, a plot of the correlation coefficient of observed and estimated discharge with respect to the number of input variables for different models was provided in Fig. 6.
||Comparison of the observed and the estimated discharge of
Elman network (No. 4)
||Correlation coefficient of observed and estimated discharge
with respect to the number of input variables for different models
As it can be seen from this Fig. 6, it confirms the previous results of the models, but some new facts are as follows:
MLP network just using two input variables outperforms the regression model.
In other words, this network just confirms the form factor and the drainage
area variables as the input variables. Therefore, these variables are suggested
as the at least data needed for flood forecasting. So, for flood forecasting
in west basin at least two input variables (Fb-A) and at most four
input variables (Fb-A-Sr-H) are suggested.
According to the plot of bfg algorithm, it can be seen that there is a remarkable improved performance with two inputs rather than one. Although, the best result is obtained using five inputs, but, it is not very different from the result of using four inputs. Also, this algorithm outperforms the nonlinear regression model.
Elman network shows a weak performance compared with MLP and the nonlinear regression model because: (a) may be, it does not agree with the regression model about all of its selected variables (No. 1 to No. 6). In this case, it wont be a good network for solving hydrological problems or (b) may be it needs more hidden neurons than MLP, or (c) some of its training parameters (for example, learning rate, momentum constant) do not have the best values.
The performance of the new nonlinear regression model for flood forecasting was very satisfactory. As the number of input variables of the regression model increases, the values of correlation coefficients increase. In other words, the regression model needs more variables to estimate flood precisely and it is difficult to recognize the best variables using the regression model. According to the results of this model, Fb-A-Sr and H variables are suggested to estimate T-year flood magnitudes.MLP network (except some weak algorithms) outperforms Elman and the regression model; also, bfg algorithm of this network is suggested to estimate flood in west basin. According to the results of this network, it can be seen that this network confirms the suggested variables of the regression model for flood forecasting and also all of its selected variables as the network input variables. In order to estimate flood precisely using MLP network and few data, it is better to apply all of the training algorithms. The regression model (in determining MLPs input variables) and MLP network (in determining the best variables for flood forecasting and estimating T-year floods) as a hybrid model showed satisfactory results. The return period variable as one of the input variables of all models was very effective on precisely flood estimation.
Elman network has a low convergence speed and needs more hidden neurons than MLP. Elman network did not outperform MLP and the regression model, so, it is not suggested to estimate T-year flood events in west basins of Urmia Lake.