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Estimation of runoff volume and sediment load is the main problem that affects the performance of dams due to the reduction in the storage capacity of their reservoirs and their effect on dam efficiency and operation schedule. The simulation models can be considered for this purpose if the continuous field measurements are not available. Soil and Water Assessment Tool (SWAT) and Water Erosion Prediction Project (WEPP) models were applied to estimate the annual runoff volume and sediment load for Duhok Dam Reservoir in north of Duhok/Iraq for the period 1988-2011. The estimated annual runoff volume varied from 2.3 to 34.7 MCM for considered period. Those values were affected by rainfall depth, intensity and runoff coefficient. The resultant annual runoff coefficient for the studied area ranged from 0.05 to 0.35 (average was 0.18) causing an average runoff volume of about 14 MCM. The results of sediment routing indicated that the values of sediment yields varied from 50 to 1400 t/km2/year depending on sub basin properties. The average annual sediment load from the whole watershed is about 120 × 10
^{3} ton. The estimated total sediment arrived to Duhok Reservoir for the considered period 1988-2011 was about 2.9 × 10
^{6} ton. The results indicate that both models gave reasonable results in comparison with measured values. Based on statistical criteria, the results of both models are close to gather.

Sediments are one of the major problems of dam operation. They reduce the storage capacity of the reservoir and they can cause serious problems concerning the operation and stability of the dam [

Duhok Dam is an earth fill dam located about 2 km north of Duhok city [^{6} m^{3}, while the maximum suspended load for the period from Dec. 1978 to Feb. 1980 was 5 kg/m^{3}. The estimated sediment load indicates that they have a significant effect of dam storage capacity. [

Previous studies concerning Duhok Reservoir dealt with runoff, sediment concentration and load or the factors affecting them are for a specific period. None of the previous studies was concerned with estimating the total sediment load accumulated after 24 years of dam operation. The objective of this research is to evaluate the total sediment load entered Duhok Dam Reservoir since its operation in 1988 till 2011 based on application of both SWAT and GeoWEPP models, also evaluation of the models performance for estimation runoff and sediment load is another objective. It is noteworthy to state that Iraq is suffering from water shortage problems now [

The study area is the watershed of Duhok Dam Reservoir. Duhok Dam is an earth fill dam located at 2 Km north of Duhok City. The dam height is 64 m, length 613 m and crest width 9 m. The storage capacity is 52 × 10^{6} m^{3} including 47.51 × 10^{6} m^{3} live storage and 4.3910^{6} m^{3} dead storage. The dam was built to serve for irrigation of agricultural areas of about 4300 hectare, supply water for Duhok City as well as recreation area. The watershed area of Duhok Dam is about 130 km^{2} bounded by the coordinates 36˚51'20''N, 37˚01'00''N, 42˚50'30''E and 43˚05'50''E. It is divided into four sub watersheds (Dolee Garmava, Sindor, Bajlor and Per Omara) as shown in ^{2}, while the slopes are between 19.8% to 25.7%. The difference geometric properties of the watersheds are given in

Watershed | Area (km^{2}) | Length (Km) | Average Slope % | Mean Elevation (m.a.s.l) |
---|---|---|---|---|

Dolee Garmava | 86.5 | 17.6 | 19.8 | 891 |

Sindor | 30.3 | 7.6 | 17.7 | 855 |

Bajlor | 8.1 | 3.1 | 23.3 | 825 |

Per Omara | 5.0 | 2.9 | 25.7 | 776 |

Soil and Water Assessment Tool (SWAT) is a continuous simulation model developed by the USDA-Agricul- tural Research Service. It is a physically based model to estimate runoff, nutrient losses, chemical and sediment transport within the watershed scale for daily time step [

The SCS curve number method was considered in this study, it is an empirical method to estimate the surface runoff based on studies of different rainfall-runoff relationships for small rural watersheds, then developed for different types of soils and land use [

where:

Q_{surf}: Accumulated runoff (mm),

R_{day}: Daily rainfall depth and

S: Retention parameter.

The retention parameter varies depending on soil type, slope, land cover and management and antecedent moisture conditions, it is equal to:

where:

CN: Curve number for normal hydrological condition.

The initial abstraction (I_{a}) is considered equal to 0.2S, so the Equation (1) becomes in the following form:

The curve number for normal moisture condition (field capacity) identified based on soil type and land use, then it is modified based on antecedent moisture condition.

The sediment load estimation in SWAT model was executed for each hydrological response unit (HRU) divided in to two phase, overland phase and channel flow. The Modified Universal Soil Loss Equation (MUSLE) (31) was considered to estimate the erosion and sediment load

where,

^{3}/s),

Then the simple form of stream power theory was applied to estimate the channel sediment routing including degradation or deposition. The channel bed and bank erosion will occur when stream flow transport capacity is greater than sediment load (coming from upstream region) at that reach and flow shear stress is greater than the stress required to detach the soil particles. While the deposition will occur in the case that sediment load is greater than transport capacity, the rate of sediment deposition depends on the fall velocity of particles.

Water Erosion Prediction Project WEPP also as SWAT model is a physically based model to predict flow, soil detachment, deposition and sediment load. It is a continuous simulation model developed by United States Departments of Agriculture and Interior. The model consider the hydrology, plant growth, flow and erosion and deposition process for hill slopes and relatively small watersheds and it was publicly released in 1995 [

The surface runoff which is based on excess rainfall is estimated by Green Ampt Mein Larson (GAML) model in the following form:

where:

The sediment load in WEPP model for both hill slope and channel flow consist of detachment, transport and deposition. The steady state continuity equation for sediment load estimation down a hill slope profile is considered in the following form:

where,

G: Sediment load (kg/s/m),

x: Distance in the flow direction (m),

^{2}),

^{2}).

For channel erosion, a steady state sediment continuity equation is also considered in the following form:

where,

x: Distance along the channel (m),

^{2}),

^{2}).

In order to apply the SWAT and WEPP models to estimate the runoff and sediment load entering Duhok Dam Reservoir from the watershed, the models were calibrated for both runoff and sediment concentration as a first step to ensure the models ability. Generally, the required input data of the both models included: description of watershed topography as a Digital Elevation Model (DEM) and daily readings of rainfall and other climate data, Duhok Dam and Duhok weather stations (the nearest stations to study area) were considered for that. In addition to climatic data, the soil type and land cover and land management as digital maps are required.

The available measured runoff data are the accumulated monthly values for the rain season 2008-2009 as presented by [^{3} m^{3} is that the predicted value were zero for both models. This difference is attributed to the fact that the total monthly rainfall at that month did not exceed 5mm, while there is a measured runoff value, so maybe there is an error in field measurements.

As far as the statistical criteria concerned it involved the determination coefficient (r^{2}), Nash-Sutcliffe model efficiency (Eff.) and index of agreement (d) where they were considered for models output evaluation. The best values obtained were 0.94 and 0.73 and 0.87 for the three parameters respectively for SWAT model, while there values were 0.87, 0.83 and 0.99 for WEPP model (^{2}) and Nash-Sutcliffe model efficiency (Eff.) for model performance evaluation were considered.

WEPP model, the effective factors on sediment load are inter-rill erodibility (Ki), rill erodibility (Kr) and critical hydraulic shear tc. The best values statistical criteria for runoff values obtained from WEPP model were 0.91, 0.77 and 0.93 for determination of the coefficient (r^{2}), Nash-Sutcliffe model efficiency (Eff.) and index of agree- ment (d) respectively (

The models were calibrated for both runoff and sediment load based on manual calibration tools by selecting the best values of the parameters effecting both runoff and sediments. The results of statistical criteria indicated acceptable values relative to the measured values for both models, although the parameters did not give a clear indication to consider which one of the model is better than the other.

Validation of the model was a necessity before its application. Due to limited available measured data for both runoff and sediments, the models were validated based on available measured annual sediment load data presented by Mohammed, 2010 [^{2}) and model efficiency (Eff.) and Index of Agreements (d) are 0.98 and o.60 and 0.84 for SWAT model and 0.84, 0.65 and 0.84 for WEPP model respectively, indicating acceptable model performance.

The soil and water assessment tool (SWAT) and Water Erosion Prediction Project (WEPP) were applied to estimate the runoff volume and sediment load from the watershed of Duhok Dam Reservoir. The considered period for simulation was from the year of dam operation in 1988 to 2011. The considered models were applied for daily simulation based on available data of daily rainfall, maximum and minimum daily temperature, wind speed, relative humidity and solar radiation. The resultant maximum and minimum annual runoff volume from the four basins were 34.7, 2.6 and 33.5, 23.3 MCM while the average value for considered period was 14.3 and 13.0 MCM for SWAT and WEPP models respectively.

To describe the variation of annual surface runoff coefficient and annual rainfall depth,

Year | Measured Sediment Load 10^{5} m^{3} | Predicted Sediment Load (SWAT) 10^{5} m^{3} | Predicted Sediment Load (WEPP) 10^{5} m^{3} | Model | Determination Coefficient (r^{2}) | Model Efficiency (Eff.) | Index of Agreement (d) |
---|---|---|---|---|---|---|---|

1997 | 1.70 | 1.41 | 1.99 | SWAT WEPP | 0.98 0.84 | 0.60 0.65 | 0.84 0.84 |

2001 | 0.23 | 0.86 | 0.48 | ||||

2003 | 1.40 | 1.34 | 0.88 |

and WEPP models respectively, this values happen for the years of rainfall depth below average yearly value (540 mm). That is attributed to the annual rainfall depth was less than the average. The successive years of relatively low rainfall depth less than the average values had affected the water content in the soil and this was reflected on the infiltration rate and runoff coefficient especially in the second year. The maximum rainfall recorded in the area was in 1992 where it reached 879 mm. For this reason, the maximum runoff coefficients of 0.33 and 0.35 for the SWAT and WEPP models respectively noticed in 1993 where the rainfall depth reached 727 mm.

A logarithmic relationship (

In which:

This equation can be easily applied for the purpose of quick estimation of annual runoff volume from the study area in case of limited available data.

The sediment concentration and total annual sediment load was also estimated for the four valleys of Duhok Dam Reservoir. The sediment load depends on rain properties, runoff volume and watershed properties. For certain watershed, the rainfall depth and intensity effects the detachment of soil particles and infiltration rate, while the runoff volume carries the detached load and pickup more sediment. The total annual load predicted by SWAT and WEPP models for the years 1988-2011 are shown in ^{5}, 0.29 × 10^{5} and 2.5, 0.22 tons from the four sub watersheds for the SWAT and WEPP models respectively. The average annual values for the considered period were about 960 and 895 ton/km^{2} for both models SWAT and WEPP. For future work, logarithmic equations are suggested to estimate the annual sediment load. The first one correlate the annual sediment load (ton/km^{2}) with yearly rainfall depth D Rainfall (mm) (

Another equation correlates the yearly sediment load (ton/km^{2}) with yearly runoff volume, D_{Runoff} (m^{3}) in the following

The determination coefficient for Equations (9) and (10) are 0.64 and 0.73 respectively.

The sediment per unit area varies between the watersheds depending on land cover, soil type and land topography. Also for the individual watershed, the detachment and transport of the particles varies from point to point. In SWAT model, the watershed was divided into a number of hydrological response units (sub-basins) that each one has common properties, while in WEPP model, the watershed divided in to flow elements which are areas of same soil properties, managements and slope. Sediment response areas in each unit or sub-basin were identified and the average annual sediment load based on SWAT model four watersheds Dolee Garmava, Sindor, Bajlor and Per Omara are shown in

Generally, based on results of both models the annual sediment load varied from 50 to 1400 ton/km^{2} as shown in ^{2} for 60% and 35% of the area of that catchment respectively. For Sindor watershed (^{2}, while the remaining 25% is contributed higher sediment load. For Bajlor watershed (^{2} and 45% between 400 - 800 ton/year/km^{2}, while for Per Omara watershed (^{2}, while 80% of the area contribute sediment concentration of 200 - 800 ton/year/km^{2}. The variation

of sediment load values within the study area reflect the effect of surface slope and the land cover effect on transported sediment. The presented maps are useful to identify the sub-basin that is feeding the flow with high sediment concentration. These areas are to be treated with different ways for soil conservation to reduce erosion from these sub-basins.

The average annual sediment load for the considered period from the whole watershed was 124.6, 116.3 ton while the total sediment load was 2.99 × 10^{6}, 2.79 × 10^{6} ton for both SWAT and WEPP models respectively, which is equivalent to about 2.4 × 10^{6} m^{3}.

Estimation of runoff and sediment load are important factors in operation schedule and maintenance of the dams. In case of limited filed measurements of runoff and sediment, simulation model can considered for this purpose. Soil and Water Assessment Tool (SWAT) and Water Erosion Prediction Project (WEPP) were applied to estimate the annual runoff and sediment load of Duhok Dam watershed in north of Iraq. The models were calibrated and validated based on available field measurements, which gave good agreements. The resultant average annual runoff and sediment load of both applied models for considered period 1988-2011 are 13.7 MCM and 120.4 × 10^{3} ton respectively. Maps of sediment yield per unit area were presented for each sub watershed to identify the sub basins of high sediment yield. Based on models results, relations between runoff-rainfall, sediment-rainfall and sediment-runoff were presented as simple tools to estimate runoff and sediment without details different data required to apply the SWAT or WEPP models.

Mohammad E. Mohammad,Nadhir Al-Ansari,Sven Knutsson, (2016) Annual Runoff and Sediment in Duhok Reservoir Watershed Using SWAT and WEPP Models. Engineering,08,410-422. doi: 10.4236/eng.2016.87038