Not many bathymetric maps are available for many lakes and reservoirs in developing countries. Usually the bathymetric mapping requires investment in expensive equipment and fieldwork, both of which are not accessible in these countries. This work demonstrates the ability to develop bathymetric map of Mosul Lake by using a digital elevation model (DEM). The depths model of the lake was designed through the use of three main stages; a coastline extraction, dataset interpolation and a triangular irregular network model. The normalized difference water index (NDWI) was used for automatic delineation of the lake coastline from satellite images. The ordinary kriging interpolation with a stable model was used to interpolate the water depths dataset. Finally a triangulated irregular network (TIN) model was used to visualize the resulting interpolation model. Calculated values of area and volume of a TIN model during 2011 were compared with values of supposed initial operation of the reservoir. The differences of water volume storage between these stages at 321 m water level was about 0.81 × 10 9 m 3, where the lake lost around 10% of storage value. Also the results of depths lake model show that the change in water storage between March and July 2011 was about 3.08 × 10 9 m 3.
Bathymetric information is essential for understanding the ecosystem of a lake [
Despite the importance of Bathymetric surveys for lakes and reservoirs, most developing countries face obstacles in this field. The cost of surveying equipment and the location of these reservoirs within unsafe regions are the biggest obstacles. The main objective of this study was to build a 3D model for the Mosul Lake bathymetric base from digital elevation data. Remote sensing data had been used to map the Mosul Lake coastline during the period of study, March and July 2011. Furthermore, a set of ordinary kriging models were tested to select the best simulation for the bathymetric dataset. The calculated values of selected bathymetric models were compared with observed field survey data to check the accuracy and observe the developing of the lake bottom during the time of reservoir operation. A 3D TIN model of Mosul Lake was designed to support decision-makers with managing the reservoir. Furthermore, models are an effective tool to understand changes in the lake environment regarding the impact of changing seasonal water storage volume.
First, confirm that you have the Mosul Dam Lake is an artificial reservoir located between latitude (36˚36'N - 36˚50'N) and longitude (42˚27'E - 42˚58'E) on the Tigris River about 60 km north of Mosul City and 80 km from the borders of Syria and Turkey
In the current study, remote sensing data were used to automatic waterline extraction for Mosul Dam Lake. A bathymetric model of the lake was constructed from a 30 m SRTM dataset. Field data survey and topographic maps were used
to verification and correction the model. The exact locations of the measurements were determined using a Garmin eTrex Legend Cx GPS.
The dataset of the Mosul Dam Lake bathymetry model analysis included 6424 geo-referenced elevation points. These points derive from, a 30 m SRTM dataset of Iraq was supplied by the U.S Army. The topographical maps were obtained from the Military Survey Office before and after the dam construction at a scale of 1:100,000 were used to check and correction the values of Mosul waterline lake. Finally, the field water depth points were measured with the Water Quality Probe (YSI 6600 V2) and standard field survey equipment used to verification the model. During field work, the elevation of the lake’s surface measured in March was 307 m.a.s.l, and in July it was almost 322 m.a.s.l. A Garmin (etrex) GPS was used to fix the geographical coordinates of field water depth points.
A coastline is a line separating land and water mass. It is a ground feature used to obtain the orientation and shape of land and water [
In the current study, the NDWI used for TM5 and ETM+ images was as follows [
where ρNIR, ρSWIR, and ρGreen are the reflectance of the near-infrared band (band 4), the short-wave infrared (band 5), and the green band (band 2), respectively.
The results of waterline extraction for Mosul Dam Lake between March 5, 2011 and July 3, 2011 are shown in
After pre-processing of the elevation and depth dataset (geographical position union and calibration between DEM and topographical map data points), these points values were merged into a database with 6424 points. The kriging or co-kriging interpolation method was used to build a digital terrain model of Mosul Lake’s bathymetry. This method is a powerful geostatistical interpolation technique based on the spatial correlation of sampled points [
where u and uα are spatial locations of an observation point and one of the neighboring data points, indexed by α, n(u) is the number of data points in the local neighborhood used for estimation, m(u) and m(uα) are the expected values (means) of Z(u) and Z(uα), and λα(u) is the kriging weight assigned to datum Z(uα) for estimation location u. In the current study, ordinary kriging type was selected to interpolate the bathymetry dataset, where this method gives the best statistical results. [
These previous complex geospatial equations were applied in ArcGIS. The geostatistical analysis option in ArcGIS was used to interpolate the dataset of Mosul Lake’s bathymetry.
There are various techniques for 3D visualizing data on digital terrain models. The regular grid surface (grid) and TIN are among the more well-known techniques [
The ordinary kriging (co-kriging) interpolation method was used to assign depth values to 84,838 geo-referenced points (
Model | ME | RMSE | ASE | MSE |
---|---|---|---|---|
Stable | 0.12 | 2.74 | 0.64 | 0.01 |
Circular | 0.18 | 2.73 | 1.12 | 0.03 |
Expoenetial | 0.21 | 2.80 | 1.15 | 0.04 |
Spherical | 0.18 | 2.71 | 1.00 | 0.03 |
Gaussian | 0.06 | 2.75 | 1.38 | 0.09 |
Most values of the accuracy error indicated that the stable ordinary kriging model is fit to predict the depth of Mosul Lake. Generally, MSE, ASE and ME were lower for the stable model than for the other models (
After having selected the model, the interpolation process included optimizing the semivariogram model of the dataset. Semivariogram is a graphical form exemplifies calculation of semivariances for different values of distance [
The classical semivariogram is calculated by using the following formula [
where γ is the calculated semivariogram of the variable v at the separation distance and direction specified by the ‘‘lag’’ vector h, the xi are the data locations and n is the number of data pairs separated by h [
where c is the total variance or sill parameter, r is the range parameter, and p is the asymptotic power-law exponent [
The relationship between modeled water depths values by ordinary kriging method and observed values for 25 locations are presented in
A three-dimensional TIN model was generated to represent the bottom surface of Mosul Dam Lake (
Model | Nugget | R2* | Range | Lags | Partial Sill |
---|---|---|---|---|---|
Stable | 0.2314 | 0.99 | 2408.1 | 12 | 38.44 |
*R2 is the coefficient of determination between predicted and measured depths.
for the TIN model. The calculation included 18,275 triangles and (10,081) nodes, and 1 m elevation tolerance with a 0.99 scale factor. The area projection of the TIN model is Transverse-Mercator with a projected coordinate system WGS-1984, UTM_Zone_38N. Value of area and volume was calculated from the TIN for the entire lake at different elevations (
The water storage volumes of Mosul Lake which were calculated by the TIN model were compared with the storage values estimated in 1968 by an IVO operational curve, (
The values of elevation vs. volume were extracted from the operation curve of Mosul Lake by using the Plot Digitizer program. The differences between the
Elevation (m.a.s.l.) | Area 2D (km2) | Volume (m3) | Elevation (m.a.s.l.) | Area 2D (Km2) | Volume (m3) |
---|---|---|---|---|---|
295 | 135.28 | 2.30 × 109 | 309 | 197.16 | 4.63 × 109 |
297 | 145.64 | 2.58 × 109 | 311 | 205.51 | 5.04 × 109 |
299 | 154.42 | 2.88 × 109 | 313 | 214.08 | 5.46 × 109 |
301 | 162.52 | 3.20 × 109 | 315 | 223.13 | 5.89 × 109 |
303 | 170.71 | 3.53 × 109 | 317 | 232.65 | 6.35 × 109 |
305 | 179.29 | 3.88 × 109 | 319 | 243.18 | 6.82 × 109 |
307 | 188.59 | 4.25 × 109 | 321 | 273.80 | 7.33 × 109 |
Elevation (m.a.s.l.) | Volume(IVO) (m3 × 109) | Volume(TIN) (m3 × 109) | Differences (m3 × 109) |
---|---|---|---|
321 | 8.14 | 7.33 | 0.81 |
319 | 7.74 | 6.82 | 0.92 |
317 | 7.07 | 6.35 | 0.72 |
315 | 6.31 | 5.89 | 0.42 |
313 | 5.84 | 5.46 | 0.38 |
311 | 5.32 | 5.04 | 0.28 |
309 | 4.90 | 4.63 | 0.27 |
307 | 4.52 | 4.25 | 0.27 |
305 | 4.05 | 3.88 | 0.17 |
303 | 3.65 | 3.53 | 0.12 |
301 | 3.21 | 3.20 | 0.01 |
299 | 2.89 | 2.88 | 0.01 |
297 | 2.58 | 2.58 | 0.00 |
295 | 2.30 | 2.30 | 0.00 |
storage volumes of the lake calculated by the TIN model and the IVO operation curve (
The current study explored the ability and certainty of extracting bathymetric map data for the Mosul Dam Lake from a digital elevation model. The study recommended an approach for monitoring volume water storage of this reservoir using remote sensing data and GIS techniques. The methodology tested proved that the ordinary kriging interpolation method is the best model that can be applied to develop bathymetric maps of this lake. Furthermore, the verification performance of this model with observed field data values confirmed the high efficiency for the selected model. This study showed that Mosul Lake has lost about 10% of its ability to store water up to 2011. This could be a result of trap efficiency, where sediment from the Tigris River is deposited behind the dam as well as in the valleys around the lake. Also, the Mosul reservoir witnesses high seasonal variation of storage water reaching to 3.08 × 109 m3 which may lead to fluctuations in environmental conditions. Although, the calculated values of depths in the deeper regions have errors, the proposed model is still highly recommended over the more expensive traditional bathymetric data collection methods such as boat and radar in such a region of the world.
Khattab, M.F.O., Abo, R.K., Al-Muqdadi, S.W. and Merkel, B.J. (2017) Generate Reservoir Depths Mapping by Using Digital Elevation Model: A Case Study of Mosul Dam Lake, Northern Iraq. Advances in Remote Sensing, 6, 161- 174. https://doi.org/10.4236/ars.2017.63012