Hydrological Impact of Ilisu Dam on Mosul Dam; the River Tigris

: The Ilisu Dam is part of the Turkish Southeastern Anatolia Project (GAP) and is the largest dam on the Tigris River in Turkey. It is located on the main river course 65 km upstream of the Syrian and Iraqi border. The Ilisu Dam watershed is the same as that of the Mosul Dam in Iraq. Sharing the same watershed with the Mosul Dam and located upstream, the Ilisu Dam will usurp most of the watershed and deprive the Mosul Dam of most of its current inﬂow. This paper presents an assessment of the hydrological impact (basically predicts changes on future inﬂow) of the Ilisu Dam on the Mosul Dam. The assessment is based on the worst-case scenario. The analyses that are employed include geographic information system (GIS) techniques and regression models, along with statistical analyses to numerate expected future impacts on the Mosul Dam’s inﬂow distribution. Results reveal that the Ilisu will have a drastic impact on the inﬂow regime of the Mosul Dam. A reduction as high as 78% of the inﬂow of the Mosul Dam may occur if the operation of the Ilisu and the Cizre Dams is conducted with no consideration of downstream hydrological and environment impacts.


General
The world's water resources are stressed due to high demand and limited availability. The need for drinking water, food production and environmental protection are increasing as the world population increases. Water scarcity problems are more challenging in arid and semi-arid countries such as the country of Iraq. Iraq depends heavily on the Twin Rivers; the Euphrates and the Tigris, which provide most of the water needs of the country. The two rivers are heavily regulated and controlled by the riparian countries (Turkey, Syria and Iraq). Iraq, being the downstream riparian, has the least control on its water supplies [1]. The water crisis in the Euphrates and the Tigris basins may be explained by two factors: the lack of collaboration among the riparian countries and the absence of basin-wide water resources management plans [1,2].
In 1977, Turkey's Southeast Anatolia Project "Guneydogu Anadolu Projesi in the Turkish Language" (GAP) was initiated on the upper parts of the Euphrates and Tigris basins (the upper Mesopotamia) [3][4][5]. Two years earlier, Turkey had constructed the Keban Dam and Syria had launched the Tabqa Dam, both on the Euphrates River. The GAP is a Turkish irrigation-agricultural scheme that includes at least 22 dams on the upper Euphrates and Tigris Rivers [6]. The most recent dam project of the GAP system is the Ilisu Dam on the Tigris River [7].
Ilisu during dry years would be diverted via the Cizre Dam. They stated that with full implementation of the Ilisu-Cizre projects, all the summer flow could be diverted before it crosses the international border to Iraq. Based on Bilgen [5] statement, it is likely that inflow to the Mosul Dam will be altered, interrupted and reduced.
Hence, it is required to re-evaluate the inflow regieme to the Mosul Dam Reservoir assuming the worst-case scenario. The worst case as defined by Merriam-Webster Dictionary involves, projecting, or providing for the worst possible circumstances or outcome of a given situation [24]. The worst-case scenario is a useful device when the planner has practically no control on future events. Since the future release from the Ilisu Dam is entirely controlled by Turkey and Turkey has committed no specified flow for the Mosul Dam, then the future operation of the latter must be altered to recognize the new realities. The scenario to consider is the flow that is reliable regardless of the flow release downstream of Ilisu Dam. The reliable flow comes from the reduced watershed left for the Mosul Dam Reservoir. The purpose of this paper is to evaluate the worst-case scenario inflow to the Mosul Dam. The "new" watershed is delineated and recalculated using "Arc Hydro Tools" within geographic information system (GIS) techniques. The generated inflow is based on analyzing historical flow data and the proposed regression models.

Study Area, Watershed Characteristics and Rainfall Distribution
The study area is located along the Upper Tigris River, mostly, within Turkey and Iraq. The study area is bordered by (36 • 25 09"-38 • 45 36") N and (39 • 15 18"-43 • 59 38") E. The watershed's area covers about 50,965 km 2 ( Figure 1). A small part of the study area is in Syria. The "Arc Hydro Tools" of the GIS was applied to delineate watersheds for the three dams (Mosul, Ilisu and Cizre). The "Arc Hydro Tools" via "Terrain Preprocessing" is employed to obtain stream definition, flow accumulation, flow direction and drainage point outlets of the catchments. The digital elevation model (DEM) of the United States Geological Survey (USGS) [25] with grid size of 30 × 30 m was employed to delineate the administrative areas, inland water, land cover and country boundaries within the study area. Results showed that the area's elevation ranges between 254 to 3622 m (AMSL) with an average of 1212 m (AMSL) as shown in Figure 1. The DEM was divided into five classes. The area between any two adjacent DEM classes was determined and the average weighted elevation for each watershed was calculated using the measurement tool of the GIS.
The Details of the average elevation for the study area are shown in Table 1. The average weighted elevation for each watershed was calculated as follows: where, E a is the weighted average elevation, m (AMSL); E i is the average DEM for each subdivision, m (AMSL); A i is the subdivision of ith sub-catchment area, km 2 ; n is total of sub-catchments. The calculated total watershed area is 50,965 km 2 . This watershed was originally designated for the Mosul Dam. Currently, it is divided between the Mosul and Ilisu Dams. The larger part of the divided watershed is allocated for the Ilisu Dam from the hydrological point of view. The total watershed's area calculated by this study is very close to the watershed's area calculated by Saleh [26]. committed no specified flow for the Mosul Dam, then the future operation of the latter must be altered to recognize the new realities. The scenario to consider is the flow that is reliable regardless of the flow release downstream of Ilisu Dam. The reliable flow comes from the reduced watershed left for the Mosul Dam Reservoir. The purpose of this paper is to evaluate the worst-case scenario inflow to the Mosul Dam. The "new" watershed is delineated and recalculated using "Arc Hydro Tools" within geographic information system (GIS) techniques. The generated inflow is based on analyzing historical flow data and the proposed regression models.

Study Area, Watershed Characteristics and Rainfall Distribution
The study area is located along the Upper Tigris River, mostly, within Turkey and Iraq. The study area is bordered by (36° 25' 09"−38° 45' 36") N and (39° 15' 18"−43° 59' 38") E. The watershed's area covers about 50,965 km 2 ( Figure 1). A small part of the study area is in Syria. The "Arc Hydro Tools" of the GIS was applied to delineate watersheds for the three dams (Mosul, Ilisu and Cizre). The "Arc Hydro Tools" via "Terrain Preprocessing" is employed to obtain stream definition, flow accumulation, flow direction and drainage point outlets of the catchments.  Terrain slope for study area as well as for each watershed was obtained using the surface function in the "Spatial Analyst Tools". The input raster was the DEM and the output raster was measured in degrees. The resulted general slope is running towards the Tigris River ( Figure 2). The total terrain slope was subdivided into two classes and ranged from 0 to 90 degrees based on natural breaks. The highest slope average is 89.2 degrees. Small pockets of the watershed have a zero slope, located around the Mosul Dam's Reservoir and Lake Hazar as shown in Figure 2. No major changes in the slope have been deducted between the total watershed and the individual sub-watersheds. The total length of the part of the Tigris River within the studied watershed is about 496.2 km with an elevation of 263 m to 976 m (AMSL). The slope of this part of the river is between 0.0007 and 0.0160 with an average of 0.005.
Rainfall distribution was obtained from a publication by the UN-ESCWA and BGR [27]. A regional annual average precipitation isohyetal map imposed on the entire watershed area. The area between any two adjacent isohyets was determined for the purpose of calculating the average regional precipitation. Details of the calculations' results are presented in Table 1. The average regional precipitation for each watershed was calculated as follows: where, P a is the average regional precipitation, mm; and P i is the average regional precipitation of the ith area (between two adjacent isohyetals). The average regional precipitations were 727, 737 and 724 mm for the whole watershed (design watershed), the Mosul Dam watershed, and the Ilisu-Cizre Dams watershed, respectively, ( Table 1).
Geosciences 2020, 10, x FOR PEER REVIEW 5 of 14 regional precipitation. Details of the calculations' results are presented in Table 1. The average regional precipitation for each watershed was calculated as follows: where, Pa is the average regional precipitation, mm; and Pi is the average regional precipitation of the i th area (between two adjacent isohyetals). The average regional precipitations were 727, 737 and 724 mm for the whole watershed (design watershed), the Mosul Dam watershed, and the Ilisu-Cizre Dams watershed, respectively, ( Table 1).

Inflow Calculations and Statistical Analysis
Observed inflow data to the Mosul Dam's Reservoir was obtained from the MoWR. The data is for a period of 30 years (October 1987 to September 2016). The inflow data were subjected to statistical analyses, mainly a probability distribution and a regression analysis. Statistical parameters (statistics) for observed inflows are presented in Table 2. The statistics include the mean, the standard deviation,

Inflow Calculations and Statistical Analysis
Observed inflow data to the Mosul Dam's Reservoir was obtained from the MoWR. The data is for a period of 30 years (October 1987 to September 2016). The inflow data were subjected to statistical analyses, mainly a probability distribution and a regression analysis. Statistical parameters (statistics) for observed inflows are presented in Table 2. The statistics include the mean, the standard deviation, the maximum, the minimum, the median, the 25th percentile and the 75th percentile of the observed data. There are several probability distributions to describe the observed data for monthly inflow estimations such as the normal, the long-normal, the Pearson Type III, the Weibull and the Gumbel's extreme values. The Weibull method [28] distribution was selected because it gave reasonable fit to the observed inflow flow data. The Weibull approach employed in this paper is based on the probability of occurrence of a flood (having a recurrence interval of T r ) in any year by calculated the probability of exceedance as follows: where, P p is the probability of exceedance; m is order or rank of the event; and N is number of events (data points). The Weibull method was applied for the monthly flows for the period of record using Equation (3). Graphs of exceedance probability for each month are presented in Figure 3. Yihdego and Webb [14] discussed the land use change and reduction in runoff using water balance model (modified tanh equation). The modified tanh equation is expressed as follows: where, Q is the predicted monthly flow; H is the median monthly base flow; RS is cumulative surplus rainfall (monthly rainfall subtracting to the monthly evapotranspiration); and V is a value to determine the shape of the tanh curve and can be computed by fitting the tanh curve to the observed data. The modified tanh equation as well as the original tanh model were derived to establish runoff rainfall relationship. The modified tanh equation intended to predict runoff changes associated with changes in land use. Therefore, Equation (4) could not account for changes in watershed area; area reduction as is the problem of the present paper. Accordingly, the modified tanh equation cannot be utilized for watersheds of this study. Thus, a regression model is more suitable to predict runoff instead of Equation (4).
Regression equations were generated to estimate monthly and annual flows based on results of Figure 3. The derived equations are based on the area of the studied watershed. The USGS [29] developed regional regression equations for estimating magnitude and frequency of runoffs for ungagged sites across some watersheds in the USA. In the same approach, the general form of the derived equations in this study is expressed as: where, Q is the estimated monthly flow, m 3 /sec; a is the coefficient of the regression model; b is the exponent of the regression model; and A is the catchment area, km 2 .
Geosciences 2020, 10, x FOR PEER REVIEW 7 of 14 where, Q is the estimated monthly flow, m 3 /sec; a is the coefficient of the regression model; b is the exponent of the regression model; and A is the catchment area, km 2 . The parameters a and b were derived for each month and listed in Table 2. The above findings were applied to predict flows to the Mosul Reservoir following the full operation of the Ilisu-Cizre scheme. Since, Equation 5 is based on one main variable (the catchment area, A); it is reasonable to apply it to any part of the studied watershed, considering the divide watershed that still has the same average DEM, slope and precipitation. The pre-Ilisu inflow is the observed inflow for the last 30 years (results from Figure 3). The generated-post-Ilisu inflow (proposed in this study) is the predicted inflow to the Mosul Dam using Equation 5 after the full operation of Ilisu-Cizre scheme. The generated post-Ilisu inflow was statistically compared with as the post-Ilisu inflow predicted by Declaration et al. [13] study.
The computed statistical differences in regression equations, observed inflow data, generatedpost-Ilisu by this study, the Declaration et al. [13] study inflows were examined using the analysis of variance (ANOVA) technique within SigmaPlot 13.5 Software. A one-way ANOVA was utilized to describe and compare the data with basic statistics for data with more than two groups. A one-way ANOVA reported the mean, standard deviation, maximum, minimum, median, 25% and 75% of the observed data. The pairwise comparisons of t-tests are calculated to compare the regression equations, observed inflow data, generated-post-Ilisu, Declaration et al. inflows using the Tukey test method. The ANOVA is considered a significant difference when the level of α is equal or less than 0.05 [30−32].

Watershed and Precipitation Distribution Analysis
The original watershed area for the Mosul Dam as listed in the design report is 54,900 km 2 [26]. This watershed area includes all the watershed of the Tigris River upstream of Mosul Dam. The Mosul Dam watershed area was calculated for this paper using ArcMap GIS is 50,965 km 2 (Figure 2). The Mosul Dam watershed area, however, is now reduced by the newly constructed Ilisu Dam. As The parameters a and b were derived for each month and listed in Table 2. The above findings were applied to predict flows to the Mosul Reservoir following the full operation of the Ilisu-Cizre scheme. Since, Equation (5) is based on one main variable (the catchment area, A); it is reasonable to apply it to any part of the studied watershed, considering the divide watershed that still has the same average DEM, slope and precipitation. The pre-Ilisu inflow is the observed inflow for the last 30 years (results from Figure 3). The generated-post-Ilisu inflow (proposed in this study) is the predicted inflow to the Mosul Dam using Equation (5) after the full operation of Ilisu-Cizre scheme. The generated post-Ilisu inflow was statistically compared with as the post-Ilisu inflow predicted by Declaration et al. [13] study.
The computed statistical differences in regression equations, observed inflow data, generated-post-Ilisu by this study, the Declaration et al. [13] study inflows were examined using the analysis of variance (ANOVA) technique within SigmaPlot 13.5 Software. A one-way ANOVA was utilized to describe and compare the data with basic statistics for data with more than two groups. A one-way ANOVA reported the mean, standard deviation, maximum, minimum, median, 25% and 75% of the observed data. The pairwise comparisons of t-tests are calculated to compare the regression equations, observed inflow data, generated-post-Ilisu, Declaration et al. inflows using the Tukey test method. The ANOVA is considered a significant difference when the level of α is equal or less than 0.05 [30][31][32].

Watershed and Precipitation Distribution Analysis
The original watershed area for the Mosul Dam as listed in the design report is 54,900 km 2 [26]. This watershed area includes all the watershed of the Tigris River upstream of Mosul Dam. The Mosul Dam watershed area was calculated for this paper using ArcMap GIS is 50,965 km 2 (Figure 2). The Mosul Dam watershed area, however, is now reduced by the newly constructed Ilisu Dam. As calculated for this paper, the area contributing flow to the Ilisu Dam is estimated as 37,064 km 2 (Figure 4a). It is about 73% of the original Mosul Dam watershed. If both the Ilisu and the Cizre Dams are jointly considered, the total watershed area that will be subtracted from the Mosul Dam watershed increased to 39,847 km 2 (Figure 4b).
Geosciences 2020, 10, x FOR PEER REVIEW 8 of 14  Table 1). According to Table 1, no statistical differences of average elevation were found between the original watershed versus the Mosul Dam watershed and the Ilisu-Cizre Dam watershed with P-value of 0.40. Hence, it is reasonable to conclude that elevations are equally distributed over the two watersheds (Ilisu and Mosul). It is concluded that elevations have no major influences that could edge the generated outflow from either watershed. The highest slope of the original watershed is between 88 to 90 degrees with an average of 89.2 degrees (Figure 2). Similar to DEM analysis, the average slope of 89.  Table 1). The difference on precipitation rates between the separated watersheds is not significant. Moreover, no statistical differences of average regional precipitation were found between the whole watershed (as one unit) and the separated watersheds with a P-value of 0.424. Hence, it is reasonable to conclude that precipitation rates are equally  Table 1). The difference on precipitation rates between the separated watersheds is not significant. Moreover, no statistical differences of average regional precipitation were found between the whole watershed (as one unit) and the separated watersheds with a p-value of 0.424. Hence, it is reasonable to conclude that precipitation rates are equally distributed over the two watersheds (Ilisu-Cizre and Mosul). It is anticipated that precipitation rates will not edge the generated discharge for either dam. In general, no major influences of elevation, terrain slope and annual precipitation rates variations on generated outflow from either watershed. It follows that the watershed area is the only controlling factor of the basin outflow. Rahi et al. [1] quantified the runoff of eight catchment areas in eastern Iraq using regression equations derived for areas in the western and southern parts of the USA. Rahi et al. [1] used several models based on the catchment area location, average terrain elevation, average annual precipitation and slope of mainstream. The authors found that the regression equations that have catchment areas as the sole variable give reasonable results with no statistically significant differences. Thus, the regression equation that was derived from Figure 3 and Table 2 (i.e., Equation 5) is proposed in this study to calculate the generated-post-Ilisu inflow.

Inflow to the Mosul Dam
The historical inflow data entering Mosul Dam Reservoir were subjected to statistical descriptive analyses as shown in Table 2. A maximum inflow of 3260 m 3 /s was recorded in 1993, while a minimum inflow of 64 m 3 /s in 1991. The highest monthly mean inflow (for the period of record) is 1493 m 3 /s and it is for the month of April, while the lowest monthly average inflow is 128 m 3 /s occurred in September. The highest 25% and 75% inflows were 1058 and 1924 m 3 /s, respectively, both recorded in April. The lowest 25% and 75% inflows were 98 and 156 m 3 /s, respectively, both recorded in September. It can be concluded that the highest and the lowest inflows (for the record period of 30 years) occurred in April and September, respectively. The parameters a and b of Equation 5 were derived for each month based on regression equations of Figure 3, as shown in Table 2.
The pre-Ilisu Dam operation flow analyses results represent the current status of the Mosul Dam inflow (Figure 6). The current mean annual inflow for the dam is about 552 m 3 /sec; the inflow that is In general, no major influences of elevation, terrain slope and annual precipitation rates variations on generated outflow from either watershed. It follows that the watershed area is the only controlling factor of the basin outflow. Rahi et al. [1] quantified the runoff of eight catchment areas in eastern Iraq using regression equations derived for areas in the western and southern parts of the USA. Rahi et al. [1] used several models based on the catchment area location, average terrain elevation, average annual precipitation and slope of mainstream. The authors found that the regression equations that have catchment areas as the sole variable give reasonable results with no statistically significant differences. Thus, the regression equation that was derived from Figure 3 and Table 2 (i.e., Equation (5)) is proposed in this study to calculate the generated-post-Ilisu inflow.

Inflow to the Mosul Dam
The historical inflow data entering Mosul Dam Reservoir were subjected to statistical descriptive analyses as shown in Table 2. A maximum inflow of 3260 m 3 /s was recorded in 1993, while a minimum inflow of 64 m 3 /s in 1991. The highest monthly mean inflow (for the period of record) is 1493 m 3 /s and it is for the month of April, while the lowest monthly average inflow is 128 m 3 /s occurred in September. The highest 25% and 75% inflows were 1058 and 1924 m 3 /s, respectively, both recorded in April. The lowest 25% and 75% inflows were 98 and 156 m 3 /s, respectively, both recorded in September.
It can be concluded that the highest and the lowest inflows (for the record period of 30 years) occurred in April and September, respectively. The parameters a and b of Equation (5) were derived for each month based on regression equations of Figure 3, as shown in Table 2.
The pre-Ilisu Dam operation flow analyses results represent the current status of the Mosul Dam inflow ( Figure 6). The current mean annual inflow for the dam is about 552 m 3 /sec; the inflow that is available 95% of the time is about 267 m 3 /sec (practically the minimum flow for the period of record). The Mosul Dam has operated well for the last three decades on this inflow. The modelled pre-Ilisu Dam was generated from a regression model (Equation (5)  When the Ilisu and Cizre Dams start their normal operation, the situation in Mosul Dam will be negatively affected. The generated post-Ilisu-Cizre operation data were obtained based on a regression model (Equation 5, using parameters a and b of 0.019 and 0.013, respectively) of the reduced watershed area ( Figure 6). Results from Figure 4b show that the area of the Tigris River basin that will contribute direct runoff to the Mosul Dam is about 22% of the original watershed (i.e. the current one). The expected mean annual inflow will be reduced to about 119 m 3 /sec and the flow that will be available 95% of the time is about 61 m 3 /sec (represented the minimum flow) (Figure 6). This future minimum inflow is equal to the environmental flow (60 m 3 /sec) that is the only flow that Turkey is committed to release post-Ilisu-Cizre operation [13]. However, it is so small that it practically turns Mosul Dam inoperable. When the Ilisu and Cizre Dams start their normal operation, the situation in Mosul Dam will be negatively affected. The generated post-Ilisu-Cizre operation data were obtained based on a regression model (Equation (5), using parameters a and b of 0.019 and 0.013, respectively) of the reduced watershed area (Figure 6). Results from Figure 4b show that the area of the Tigris River basin that will contribute direct runoff to the Mosul Dam is about 22% of the original watershed (i.e. the current one). The expected mean annual inflow will be reduced to about 119 m 3 /sec and the flow that will be available 95% of the time is about 61 m 3 /sec (represented the minimum flow) ( Figure 6). This future minimum inflow is equal to the environmental flow (60 m 3 /sec) that is the only flow that Turkey is committed to release post-Ilisu-Cizre operation [13]. However, it is so small that it practically turns Mosul Dam inoperable.
The observed mean monthly pre-Ilisu and generated-post-Ilisu inflows to the Mosul Dam were compared to inflows predicted and reported by Declaration et al. [13] (Figure 7). The latter is about 33% of the current observed flow. Not only the flow is reduced, there is zero flow that lasts for four months (June to September). The zero flow is a result of the whole flow was diverted away within Turkey [13]. This result is an indication to the fact that it is quite possible that Turkey may divert more water and the zero flow may last longer than the four months mentioned by Declaration et al. [13] study.
Geosciences 2020, 10, x FOR PEER REVIEW 11 of 14 months (June to September). The zero flow is a result of the whole flow was diverted away within Turkey [13]. This result is an indication to the fact that it is quite possible that Turkey may divert more water and the zero flow may last longer than the four months mentioned by Declaration et al. [13] study. The generated-post-Ilisu is the mean monthly inflow following the operation of Ilisu and Cizre Dams as predicted by this paper; The the mean monthly inflow following the operation of Ilisu and Cizre Dams as predicted by Declaration et al. [13].
The analysis of variance (ANOVA) technique reported the mean and standard deviation of 552, 121, and 371 m 3 /sec, and 443, 97 and 409 m 3 /sec for pre-Ilisu, generated-post-Ilisu and the predicted by Declaration et al. [13] study, respectively. The pairwise compression using Tukey test indicated that there is a significant difference between generated-post-Ilisu versus post-Declaration-Ilisu inflows with P-value of 0.015. It should be indicated that the Declaration-post Ilisu is a release from the Ilisu Reservoir according to a specific scenario outlined by the authors [13]. This release is quite different from the inflow from the reduced watershed which is the subject matter of this paper. This difference between the two approaches may explain the significant statistical difference between the The pre-Ilisu is the observed inflow for the last 30 years; The generated-post-Ilisu is the mean monthly inflow following the operation of Ilisu and Cizre Dams as predicted by this paper; The the mean monthly inflow following the operation of Ilisu and Cizre Dams as predicted by Declaration et al. [13].
The analysis of variance (ANOVA) technique reported the mean and standard deviation of 552, 121, and 371 m 3 /sec, and 443, 97 and 409 m 3 /sec for pre-Ilisu, generated-post-Ilisu and the predicted by Declaration et al. [13] study, respectively. The pairwise compression using Tukey test indicated that there is a significant difference between generated-post-Ilisu versus post-Declaration-Ilisu inflows with p-value of 0.015. It should be indicated that the Declaration-post Ilisu is a release from the Ilisu Reservoir according to a specific scenario outlined by the authors [13]. This release is quite different from the inflow from the reduced watershed which is the subject matter of this paper. This difference between the two approaches may explain the significant statistical difference between the post-Ilisu flows mentioned earlier. The mean annual cumulative inflow to the Mosul Reservoir in the case of generated-post-Ilisu (this study) is 4.6 bcm: a little more than the dead storage of Mosul Reservoir. This paper findings stress that, unless the two riparian countries (Turkey and Iraq) reach a mutual understating and share the water resources of the Tigris River reasonably, Iraq may face the severe reduction in inflow rates to the Mosul Dam (i.e. the worst scenario flow outlined throughout the paper). The flow that is reliable to the Mosul Dam is only 22% of the observed current flow. Furthermore, the flow adapted by this paper may be less than the environmental flow committed by Turkey (60 m 3 /sec) for some months of the year.

Conclusions
The Southeastern Anatolia Project (Turkish Language acronym GAP) is a Turkish water resources project that is initiated and built in the upper Euphrates and Tigris basins (upper Mesopotamia). The scheme includes at least 22 dams on both rivers. The latest of these dams is the Ilisu Dam. The Ilisu Dam is located on the Tigris River, 65 km upstream from the border line between Turkey and Syria. The dam is intended for hydroelectric power generation and will be followed by another regulator dam (Barrage) at Cizre. The latter is used for irrigation purposes. Cizre Dam is to be constructed 45 km downstream of Ilisu Dam and about 20 km upstream of the border line between Turkey and Iraq.
The Ilisu Dam, being upstream from the existing Mosul Dam of Iraq, will drain the majority of the Mosul Dam's watershed. The original designed Mosul Dam watershed (area is 54,900 km 2 ) will be reduced to about one fifth of the total area. The original Mosul Dam's watershed area as calculated for this paper is 50,965 km 2 . The area contributing flow to the Ilisu Dam as calculated by this paper is about 37,064 km 2 . If both Ilisu and Cizre Dams are jointly considered, the total watershed area that will be subtracted from Mosul Dam Watershed is about 39,847 km 2 . A point should be clarified regarding the stability dam is that recent information indicates that the dam stability has been improved considerably; that the dam reached its sixth-highest pool of record in April 2019 and showed no signs of distress.
The worst-case scenario approach was employed to estimate the reduction of Mosul's Dam flow resulting from the full operation of the Ilisu-Cizre system. The reduced inflow was calculated using a regression model based on observed flow data of 30 years. Results of the regression model show that the inflow to the Mosul Dam will be reduced to about 22% of the current flow assuming a full operation of the Ilisu-Cizre system. The cumulative flow for the whole year in the average will be about 4.6 bcm: a little more than the dead storage of Mosul Reservoir. The expected mean annual inflow will be reduced to about 119 m 3 /sec; the flow that will be available 95% of the time is about 61 m 3 /sec. The mean monthly inflow was generated and found, for some months, to be less than the environmental flow committed by Turkey.

Author Contributions:
The performing data analysis, providing required materials, ideas, formulation of overarching research aims, editing of the text; data/evidence collection were performed by A.-S.T.A.-M. and K.A.R.; reviewing similar studies and acquiring required data was the task of W.K.L. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.