3.1. Rainfall
The difference in annual rainfall from TRMM and RFE is presented in
Figure 2. The bar chart demonstrates that TRMM gives for most years a slightly higher rainfall over the Nile basin than RFE. The average value for the period 2006–2010 is 647 mm for TRMM and 600 mm for RFE. This is a difference of 47 mm or 8%. While this difference seems reasonable, the consequence of 8% difference at an average rainfall volume of 2013 km
3 is a potential error of 161 km
3/yr, which is two times the historic Nile flow at Dongola. This example demonstrates the need for a high accuracy in rainfall products. Because the official WMO rain gauge network is limited in number, and both TRMM and RFE have advantages and disadvantages, it is difficult to favor one of the rainfall products without in-depth research. For pragmatic reasons, the pixel values of the two rainfall products were linearly averaged. Ensemble rainfall products based on Earth observation data will likely lead the way forward to obtain reliable rainfall data layers. The new Climate Hazard Group IR Precipitation Station (CHIRPS) rainfall product is an example of an ensemble product based on various interpolation schemes to create spatially continuous grids from raw point data based on climatology, satellite measurements and ground precipitation observations from a variety of sources (Funk
et al., [
49]).
The average rainfall of the two products for the period 2005–2010 is 624 mm/yr (2013 km
3/yr for a basin area of 3,229,038 km
2). The FAO-Nile report (Hilhorst
et al., [
4]) gives an average rainfall volume of 2008 km
3/yr for a basin area of 3,170,418 km
2, which computes to 633 mm/yr, and which is close to the value used here (deviation is +1.4%). The FAO irrigation potential study mentions a rainfall of 615 mm/yr (deviation is −1.4%) for an area of 3,112,369 km
2 or 1914 km
3/yr (FAO, 1997). Kirby
et al. estimated the Nile basin rainfall volume to be 2,043 km
3/yr (627 mm/yr; deviation is 0.5%) using data from the Climate Research Unit at the University of East Anglia (CRU TS 2.10) covering the period 1901–2002. Karimi
et al. [
50] summarized the water balance of the Nile, and estimated the total rainfall for the wet year 2007 as 2045 km
3/yr, which is plausible for an above-average rainfall volume. Hence, the estimates appear to correspond closely.
Figure 2.
Annual rainfall of the Nile as estimated by the Tropical Rainfall Measurement Mission (TRMM) and RainFall Estimates (RFE) open-access rainfall products.
Figure 2.
Annual rainfall of the Nile as estimated by the Tropical Rainfall Measurement Mission (TRMM) and RainFall Estimates (RFE) open-access rainfall products.
The consistency between the TRMM and RFE rainfall products for all 15 sub-basins is shown in
Figure 3. The scatterplot demonstrates that the overall agreement is acceptable, but that large differences in local rainfall occur (RMSE is 153 mm/yr). The largest differences in absolute rainfall amounts occur over the equatorial Nile zone. The rainfall rates over Lake Victoria and the Victoria Nile sub-basins are frequently more than 250 mm/yr different: the difference of 250 mm/yr is unlikely. Rainfall estimates of the Blue Nile sub-basin also seem to have unreasonable differences. It is therefore concluded that more research is needed to validate local rainfall products from Earth observation data. The success of calibration will increase if the data are exposed to a downscaling procedure first. Examples of calibrating downscaled rainfall products are provided in Duan and Bastiaanssen ([
51]) and Hunink
et al. ([
52]).
Figure 3.
Annual rainfall data from TRMM 3B43 and FEWS-NET RFE presented by sub-basin (n = 15) for the period 2005–2010 (n = 6).
Figure 3.
Annual rainfall data from TRMM 3B43 and FEWS-NET RFE presented by sub-basin (n = 15) for the period 2005–2010 (n = 6).
The rainfall regimes differ significantly across the different sub-basins, and rainfall evaluation should actually focus on the sub-basins only (see
Table 1). The humid tropical Kagera, Lake Victoria, Semiliki and Lake Albert and Victoria Nile basins all show substantial amounts of rainfall throughout the year, with their peaks occurring during March and April (P > 100 mm/month). The central part of the Nile basin receives rainfall during June, July and August and is relatively dry during the period November to March. The area downstream of the Blue Nile sub-basin receives substantially lower rainfall amounts. Their access to water resources depends entirely on the rainfall surplus (P-ET) from the upstream basins.
Table 1.
Monthly rainfall from the combined TRMM and RFE products averaged for the period 2005–2010 by sub-basin.
Table 1.
Monthly rainfall from the combined TRMM and RFE products averaged for the period 2005–2010 by sub-basin.
Sub-Basin | Area (km2) | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | Total (mm) |
---|
Main Nile 1 | 39.896 | 9 | 10 | 5 | 4 | 2 | 0 | 0 | 0 | 0 | 3 | 2 | 6 | 40 |
Main Nile 2 | 199.564 | 2 | 2 | 2 | 2 | 2 | 0 | 0 | 0 | 0 | 0 | 1 | 2 | 13 |
Main Nile 3 | 743.913 | 1 | 0 | 0 | 1 | 2 | 3 | 21 | 31 | 8 | 1 | 0 | 0 | 70 |
Tekezze-Atbara | 231.492 | 3 | 2 | 9 | 20 | 31 | 58 | 150 | 155 | 74 | 19 | 3 | 2 | 524 |
Main Nile 4 | 35.338 | 0 | 0 | 0 | 2 | 7 | 5 | 31 | 51 | 19 | 4 | 0 | 0 | 121 |
Blue Nile | 307.262 | 6 | 11 | 22 | 36 | 87 | 138 | 231 | 225 | 140 | 63 | 14 | 6 | 978 |
Lower White Nile | 237.429 | 1 | 1 | 1 | 13 | 31 | 68 | 131 | 129 | 83 | 54 | 4 | 0 | 517 |
Bahr el Ghazal | 549.714 | 4 | 6 | 19 | 47 | 78 | 107 | 149 | 176 | 124 | 70 | 14 | 3 | 795 |
Sudd | 167.354 | 8 | 12 | 32 | 76 | 118 | 135 | 148 | 160 | 137 | 103 | 32 | 7 | 968 |
Baro-Akobo-Sobat | 230.368 | 23 | 22 | 50 | 94 | 130 | 119 | 140 | 140 | 134 | 111 | 62 | 27 | 1051 |
Albert Nile-Bahr al Jabal | 80.432 | 18 | 41 | 79 | 126 | 143 | 98 | 119 | 132 | 140 | 114 | 75 | 36 | 1121 |
Victoria Nile | 86.192 | 39 | 65 | 111 | 142 | 134 | 68 | 92 | 105 | 131 | 123 | 98 | 58 | 1166 |
Semliki-L.Albert | 70.646 | 55 | 90 | 121 | 109 | 99 | 50 | 54 | 93 | 122 | 120 | 118 | 64 | 1095 |
Lake Victoria | 191.317 | 104 | 105 | 163 | 167 | 136 | 53 | 45 | 58 | 88 | 101 | 140 | 125 | 1285 |
Kagera | 58.115 | 98 | 133 | 142 | 128 | 86 | 19 | 14 | 26 | 57 | 74 | 122 | 89 | 986 |
3.2. Actual Evapotranspiration
The application of the SSEBop model to the African continent is relatively new. Since 2012, ET anomaly products have been operationally served for various regions of the world at
http://earlywarning.usgs.gov/fews/. While the U.S. Geological Survey (USGS) Earth Resources Observation and Science (EROS) Center is in the process of preparing an operational ET service (absolute magnitudes in addition to the already served anomalies) for Africa in line with the RFE rainfall products, the ET data have not been formally released, and are available only for special ET validation and water balance studies, such as presented in this paper, in Alemu
et al. ([
53]) and Senay
et al. ([
54]). SSEBop has been validated for locations in the US using flux towers (Senay
et al., [
34]; Velpuri
et al., [
55], Singh
et al., [
56]), water balances (Senay
et al., [
57]; Velpuri
et al., [
55]) and lysimetric observations in the semi-arid Texas High Plains (Gowda
et al., [
58]; Senay
et al., [
59]). In addition, Senay
et al. ([
57]) described the comparison of ET layers between the earlier version of the algorithm (SSEB) and the more detailed output data computed with the Mapping EvapoTranspiration at high Resolution and Internalized Calibration (METRIC) model (Allen
et al., [
60]). METRIC is the American version of the Surface Energy Balance Algorithm for Land (SEBAL) model (Bastiaanssen
et al., [
61]). The METRIC comparisons for the State of Idaho, US, revealed a strong correlation (r
2 = 0.90) for elevations less than 2000 m. The thermodynamic modelling of the land surface fluxes in METRIC is known to be substantially more advanced compared to SSEBop. The SSEBop algorithm is an improved version of the SSEB, which is capable of handling a range of elevations (Velpuri
et al., [
55]). Furthermore, the simplification of the parameterization of the surface energy balance in SSEBop is justifiable and required in order to cover the African continent in an operational mode. Comparison studies for Africa will provide more insights into the absolute differences and agreements, and this paper contributes to that.
Karimi
et al. ([
35,
50]) report results of the ETLook model developed by Bastiaanssen
et al. ([
33]) for water balance and water accounts of the Nile basin during 2007. The total ET was estimated to be 2014 km
3/yr during the 2007 above-average rainfall year. The availability of the 1 km × 1 km ETLook data set for 2007, as well as of the equatorial Nile for the years 2005 and 2010 (Immerzeel
et al., [
62]), provided a great opportunity for comparison with SSEBop output data. A bias correction factor of 1.12 appears to be required. This bias can be related to hot and cold pixel computations with SSEBop that have been strongly simplified as compared to the original work of Bastiaanssen
et al. [
63] related to the selection of surface temperature end members. There is a single overall calibration coefficient in SSEBop that represents elements of the crop coefficient approach of FAO and the maximum values of evaporative fraction that are not necessarily 1.0 due to certain small values of sensible heat flux (H) over wet surfaces. This coefficient is ideal for bias corrections. The overall scatter in the spatial data of ET is higher (R
2 = 0.65) than observed for the open-access rainfall products (R
2 = 0.93). The larger scatter implies that the spatial variability of ET patterns in ETLook and SSEB are larger than for rainfall. This difference can partially be ascribed to the higher spatial resolution (1 km × 1 km) of the ET product that generates more spatial contrast. Ground truth data are necessary to independently validate the ET products from Earth observations. This validation can best be done with the help of flux towers at a few locations, and with accurate water balances of paired catchments.
After this bias correction of 1.12, the longer term average actual ET for the period 2005–2010 was computed with SSEBop as 1863 km3/yr. This is the actual ET rate that occurs due to radiative and advective energy. The additional energy due to net radiation increment of wet leaves is not included. After inclusion of the additional “interception energy”, the longer term total actual ET value totaled 1987 km3/yr, which is 6.7% more than the ET under dry surface conditions. The term “total ET” is used to express the inclusion of this extra interception energy. The lowest total ET of 1851 km3/yr occurred during the year 2010, while 2007 had the highest total ET volume (2142 km3/yr). The coefficient of variation of the 6 years analyzed is 6.2% only, and this indicates a temporary stable situation of actual ET for the whole Nile basin. The temporal stability can be ascribed to the large area of desert with negligible ET values.
Table 2.
Monthly actual evapotranspiration (including interception) from the calibrated SSEBop model (mm/month). The update is based on a bias factor and a correction term for “interception energy” during periods when the land surface contains a water film and net radiation is temporarily enhanced
Table 2.
Monthly actual evapotranspiration (including interception) from the calibrated SSEBop model (mm/month). The update is based on a bias factor and a correction term for “interception energy” during periods when the land surface contains a water film and net radiation is temporarily enhanced
Sub-Basin | Area (km2) | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | Total (mm) |
---|
Main Nile 1 | 39,896 | 13 | 21 | 36 | 49 | 46 | 60 | 76 | 75 | 45 | 24 | 15 | 11 | 472 |
Main Nile 2 | 199,564 | 3 | 4 | 6 | 8 | 8 | 9 | 10 | 10 | 7 | 4 | 3 | 2 | 74 |
Main Nile 3 | 743,913 | 1 | 1 | 1 | 1 | 3 | 10 | 28 | 30 | 11 | 5 | 3 | 1 | 97 |
Tekezze-Atbara | 231,492 | 19 | 15 | 15 | 16 | 23 | 53 | 79 | 79 | 70 | 44 | 23 | 18 | 453 |
Main Nile 4 | 35,338 | 2 | 2 | 3 | 3 | 7 | 18 | 51 | 61 | 25 | 5 | 1 | 1 | 180 |
Blue Nile | 307,262 | 45 | 38 | 37 | 39 | 52 | 72 | 89 | 87 | 90 | 83 | 57 | 47 | 737 |
Lower White Nile | 237,429 | 35 | 29 | 25 | 22 | 34 | 62 | 95 | 100 | 80 | 64 | 38 | 34 | 617 |
Bahr el Ghazal | 549,714 | 51 | 48 | 53 | 50 | 68 | 81 | 93 | 95 | 88 | 80 | 62 | 53 | 823 |
Sudd | 167,354 | 97 | 87 | 86 | 79 | 102 | 105 | 104 | 103 | 109 | 119 | 114 | 103 | 1,209 |
Baro-Akobo-Sobat | 230,368 | 87 | 70 | 65 | 70 | 82 | 82 | 86 | 82 | 92 | 106 | 99 | 91 | 1,012 |
Albert Nile-Bahr al Jabal | 80,432 | 85 | 72 | 88 | 87 | 105 | 100 | 94 | 93 | 105 | 114 | 113 | 88 | 1,144 |
Victoria Nile | 86,192 | 93 | 77 | 90 | 89 | 91 | 85 | 82 | 84 | 90 | 101 | 99 | 86 | 1,067 |
Semliki-L.Albert | 70,646 | 92 | 82 | 90 | 86 | 86 | 83 | 78 | 78 | 84 | 88 | 90 | 87 | 1,023 |
Lake Victoria | 191,317 | 96 | 90 | 102 | 95 | 89 | 82 | 80 | 79 | 83 | 86 | 88 | 85 | 1,056 |
Kagera | 58,115 | 77 | 67 | 82 | 83 | 82 | 73 | 68 | 69 | 68 | 71 | 79 | 71 | 891 |
The monthly ET layer for each sub-basin is presented in
Table 2. These values represent a mix of agro-ecosystems. The sub-basin with the highest ET per unit of land is the Sudd (1209 mm/yr) followed by the Albert Nile—Bahr Al Jabal (1144 mm/yr). Both sub-basins host extensive wetlands and tropical forests, and have a rich biodiversity. Mohamed
et al. ([
64]) estimated the ET rates of a 38,600-km
2-wide Sudd area to be 1636 mm/yr. The areas prone to floods were included in their analysis. The central part of the Sudd (shown in
Figure 4) displays actual ET rates between 1500 and 2000 mm/yr, similar to the ET values reported by Mohamed
et al. ([
64]). The spatial coverage of the entire Sudd sub-basin is 167,354 km
2, which is substantially larger than the area used by Mohamed
et al. ([
64]), and includes the pastures surrounding the wetlands. This difference in area provides a logical explanation for the lower ET value shown in
Table 2 (1209 mm/yr). The lowest ET by sub-basin occurs in northern Sudan and upper Egypt where the majority of the agro-ecosystem is desert (74 mm/yr). The impact of irrigation on the sub-basin averaged ET is clear when considering the sub-basin average ET value for the Main Nile 1 sub-basin (472 mm/yr). This value is roughly an average value for a landscape consisting of double cropping systems in the Nile delta (ET of 1000–1500 mm/yr) and desert land (ET of 10–100 mm/yr).
Figure 4.
Distribution of the annually accumulated actual evapotranspiration across the Nile basin averaged for the period 2005–2010. The pixel size is 1 km. The data are based on SSEBop being calibrated against ETLook and adjusted for interception energy.
Figure 4.
Distribution of the annually accumulated actual evapotranspiration across the Nile basin averaged for the period 2005–2010. The pixel size is 1 km. The data are based on SSEBop being calibrated against ETLook and adjusted for interception energy.
Hence, the average total ET for the Nile basin for the period 2005–2010 is 1987 km
3/yr. The FAO-Nile program estimated the total ET to be 1991 km
3/yr (Hilhorst
et al., [
4]), being a difference of less than 0.2%. Their methodology is based on a classical water balance calculation for sub-basins. Actual ET over rainfed areas is assumed to be equal to the reference ET
0 when there is enough water stored in the soil to allow actual ET fluxes to be equal to ET
0. During dry periods, a soil moisture reduction term is used. The land use map for irrigated areas was derived from AQUASTAT, and it was assumed that actual ET equaled ET
0 after correction with a crop coefficient that reflects pristine growing conditions, without any moisture or salinity stress, following Allen
et al. ([
65]). Open water and wetland evaporation was estimated from reference evapotranspiration and calibrated through closure of the water balance. Unfortunately, the boundaries of the sub-basins reported by Hilhorst
et al. ([
4]) are not identical to the official boundaries of the sub-basins outlined by the Nile Basin Initiative and used in this paper. It is therefore more useful to compare ET expressed as a rate per unit of land, and not by volume. Except for Bahr el Ghazal and Sudd, the differences are within 15% (see
Table 3) and the RMSE is 295 mm/yr. The overall correlation of R
2 = 0.92 between the FAO-Nile ET by basin and the aggregated ET value on the basis of 1 km × 1 km pixels is encouraging, despite the differences between the geographical coverage.
Kirby
et al. ([
66]) estimated the ET (referred to as “water use”) to be 1828 km
3/yr, with the majority of the water being evapotranspired by grassland (937 km
3/yr), followed by woodland and other (563 km
3/yr), rainfed agriculture (264 km
3/yr), and irrigated agriculture (65 km
3/yr). Remarkably, they did not report on the evaporation from open water bodies, and that is one of the reasons that their total value of 1828 km
3/yr is lower than the 1987 km
3/yr found in this study. The Kirby
et al. model calculates actual ET based on potential ET and water availability. Potential ET acts as an upper boundary for actual ET. Storage of water in soil and on the surface is calculated using a soil moisture bookkeeping procedure and water balance equation. Sub-basins identified as Main Nile 1 and 2 are the irrigated areas in Egypt. The average ET of these sub-basins for the period 2005–2010 is 35 km
3/yr, and this number compares well with the 35 km
3/yr of ET in agriculture mentioned in the National Water Resources Plan for Egypt (MWI, [
67]). In addition, there is ET from surface evaporation (2.5 km
3/yr), fallow land 0.9 km
3/yr, fish ponds (0.2 km
3/yr), and municipal and industrial use (2.5 km
3/yr). Some of these national-scale non-agricultural ET values occur, however, outside the Nile basin. So, this ET number officially endorsed by the Egyptian authorities agrees well with the Earth observation estimates.
Table 3.
Comparison of longer term ET volumes (km3/yr) and fluxes (mm/yr) estimated by FAO-Nile (1960–1990) and the adjusted USGS EROS SSEBop model (2005–2010). The deviation is based on the ET flux values.
Table 3.
Comparison of longer term ET volumes (km3/yr) and fluxes (mm/yr) estimated by FAO-Nile (1960–1990) and the adjusted USGS EROS SSEBop model (2005–2010). The deviation is based on the ET flux values.
FAO-Nile | Adjusted SSEBop Model |
---|
Description | Area | ET | ET | Description | Area | ET | ET | Dev |
---|
(km2) | (km3/yr) | (mm/yr) | (km2) | (km3/yr) | (mm/yr) | (%) |
---|
Main Nile d/s Atbara | 877,866 | 108.8 | 124 | Main Nile 1,2,3 | 98,3375 | 105.7 | 107 | 13.3 |
Atbara | 237,044 | 94.1 | 397 | Tekezze-Atbara | 231,492 | 104.8 | 453 | −14.0 |
Main Nile d/s Khartoum | 34,523 | 7.3 | 211 | Main Nile 4 | 35,338 | 6.4 | 180 | 14.7 |
Blue Nile | 308,198 | 266.0 | 863 | Blue Nile | 307,262 | 226.4 | 737 | 14.6 |
White Nile | 260,943 | 144.5 | 554 | Lower White Nile | 237,429 | 146.4 | 617 | −11.4 |
Bahr el Ghazal & el Arab | 606,428 | 454.1 | 749 | Bahr el Ghazal - Sudd | 717,069 | 654.7 | 913 | −21.9 |
Pibor-Akabo-Sobat | 246,779 | 223.8 | 907 | Baro-Akobo-Sobat | 230,369 | 233.2 | 1012 | −11.6 |
Bahr el Jebel | 136,400 | 163.1 | 1,196 | Albert Nile-Bahr - al Jabal | 80,433 | 92.0 | 1144 | 4.3 |
Kyoga-Albert | 197,253 | 221.6 | 1,124 | Victoria Nile Semliki - L.Albert | 156,839 | 164.2 | 1047 | 6.8 |
Lake Victoria basin | 264,985 | 307.5 | 1,160 | Lake VictoriaKagera | 249,433 | 253.8 | 1018 | 12.3 |
Total and average | 3,170,419 | 1990.8 | 628 | | 3,229,039 | 1987.6 | 616 | |
3.3. Water Balance
The long-term average flows in the River Nile are measured and published in several sources. Based on data from Sutcliffe and Parks [
18] and others, river flows at various points in the Nile basin were compiled before by Dai and Trenberth [
68], Awulachew
et al. [
69], El-Shabraway [
70] and Johnston [
71]. The water balance for every sub-basin can also be computed from the Earth observation-based P, ET and ΔS data. It is a unique situation to have spatially distributed ET data. In combination with flow data, the ET data will facilitate the understanding of the water balance of the Nile.
Congruency between P, ET and ΔS values can be obtained from the mass balance. The inflow and outflow across sub-basins can be determined by accounting for P, ET and ΔS. Stream discharge gauges were used to measure flows in the main river courses. Outflow of sub-basins does not follow the main river course only. Inter-basin transfer also occurs through floods, smaller ungauged streams, groundwater flow and irrigation canals. The large flood plain in southern Sudan, stretching from the west (Bahr al Ghazal), across the center (Sudd) to the east (Baro Akabo Sobat), transfers large amounts of water between sub-basins when the area is submerged. Ungauged streams traverse the hilly topography and undulating terrain of the Bahr al Ghazal and Victoria Nile. The Gezira irrigation system in Sudan is located in the lower White Nile sub-basin, and receives water released by the Roseires reservoir in the Blue Nile sub-basin. Irrigation is thus an essential inter-basin transfer process. Groundwater flow occurs in areas with strong groundwater recharge. A map with annual recharge rates of 100–400 mm/yr in the Nile basin was provided by MacAllister
et al. [
72] based on earlier work of Bonsor
et al. [
73]. The greatest recharge occurs in the Blue Nile, Sudd and Bahr al Ghazal. These areas are conspicuous in
Figure 5. Areas with P-ET > 700 mm/yr occur in the Blue Nile sub-basin at the highlands of Ethiopia and Kenya. Values of P-ET also seem to be systematically higher in the southwestern part of Lake Victoria than at the eastern side of the lake. All the blue areas with P > ET in
Figure 5 are net water producing areas. Besides the typical water towers at higher elevations, there are also vast areas in north Sudan that produce a thin layer of water (P-ET > 50–100 mm/yr), which volume-wise, contributes substantially to groundwater recharge and river flow (commonly via the groundwater flow system and baseflow).
Excess water from the production areas are conveyed via streams and aquifers to the lower parts of the basin. The largest water consumers (
i.e., most negative P-ET pixel values) are (i) lakes and reservoirs, (ii) wetlands and (iii) irrigated areas. The flooded area of the Sudd appears dominantly in
Figure 5 due to a high percentage of permanent open water bodies and swamps. The wetland areas in the Bahr al Ghazal and Sobat also appear to be vast, although flood duration might be shorter than in the Sudd; due to shorter flood seasons, the ET rates in
Figure 4 do not exceed 1400 mm/yr. The irrigation systems in Sudan are clearly visible, but P-ET is higher in absolute terms than for Egypt where the role of rainfall is basically excluded. The Kenana irrigation system located at the right bank of the White Nile near the town of Rabak is an exception, with rates of P-ET < 1000 mm/yr due to commercial monoculture sugarcane plantations. Egypt and the Sudd wetlands have large areas where the ET exceeds P by more than 1000 mm/yr, hence a substantial net withdrawal must occur.
Figure 5.
Water production areas (P > ET) and net water consumption areas (ET > P) for the Nile basin over the period 2005–2010. Lateral transport of water occurs from positive to negative areas, also non-conventionally via floods and groundwater flow
Figure 5.
Water production areas (P > ET) and net water consumption areas (ET > P) for the Nile basin over the period 2005–2010. Lateral transport of water occurs from positive to negative areas, also non-conventionally via floods and groundwater flow
Congruency of P, ET and ΔS was verified by comparing accumulated P-ET values from upstream to downstream with outflow measured at discharge gauges. The accumulation of P-ET is zero at the water divide and increases when going downstream along the river course trajectory. The accumulation of P-ET reverses after passing flood plains and other points of substantial withdrawals. The closure errors between the measured flow at discharge gauges and P-ET for each sub-basin are ascribed to ungauged inter-basin transfer processes discussed before. Maps of groundwater recharge were used to estimate whether the inter-basin-transfer is of surface or groundwater origin. The results are provided in
Table 4. It is estimated that the lower White Nile is a net receiver of surface and groundwater from floods and groundwater seepage in the southern part of the sub-basin. Bahr al Ghazal and the Sudd receive inter-basin transfer during the flood season. Lake Victoria and the Victoria Nile discharge excess water through ungauged streams and by means of deep percolation towards the flood plain areas in southern Sudan. Inter-basin transfer in Egypt happens by leakage towards the Nubian Sandstone and Mohra aquifers, among others (Shahin, [
17]; Elsawwaf
et al., [
74]). Surface water resources are withdrawn and transferred to newly reclaimed desert areas outside the Nile basin such as in Sinai, North Coast, Toshka and Oweinat. The net result of all these exchanges is that 5 km
3/yr of groundwater leaves the Nile basin by inter-basin transfer in addition to a surface water amount of 2 km
3/yr. The average outflow to the Mediterranean Sea through the Nile branches and drainage canals of the Nile delta is estimated to be 14 km
3/yr. The Egyptian National Water Resources Plan reports 12 km
3/yr (drainage: 11.7 km
3/yr; rivers 0.2 km
3/yr). Faures
et al. [
75] estimated that 12.5 km
3/yr drains to the sea and lakes, 1.2 km
3/yr directly via the rivers. The agreement between the estimated outflow in the main river course of each sub-basin and the flow measurements is presented in
Figure 6. The RMSE is 3 km
3/yr without any bias for the 15 discharge stations and the correlation is excellent (R
2 = 0.98).
Table 4.
Annual water balance by sub-basin, averaged for the period 2005–2010. Inter-basin transfer is estimated from closure of the water balance of each sub-basin.
Table 4.
Annual water balance by sub-basin, averaged for the period 2005–2010. Inter-basin transfer is estimated from closure of the water balance of each sub-basin.
SB Name | Inflow | P | ET+I | Net GW interbasin | Net SW interbasin | ΔS | Outflow |
---|
| (km3/yr) | (km3/yr) | (km3/yr) | (km3/yr) | (km3/yr) | (km3/yr) | (km3/yr) |
---|
Main Nile 1 | 36 | 2 | 19 | 4 | 1 | −0.09 | 14 |
Main Nile 2 | 55 | 3 | 16 | 4 | 1 | −0.22 | 36 |
Main Nile 3 | 79 | 51 | 71 | 4 | 1 | 0.10 | 55 |
Tekezze-Atbara | 0 | 121 | 105 | 1 | 2 | 1.19 | 12 |
Main Nile 4 | 87 | 4 | 6 | 4 | 2 | −0.07 | 79 |
Blue Nile | 0 | 299 | 237 | 5 | 6 | 1.54 | 50 |
Lower White Nile | 25 | 122 | 141 | −11 | −7 | −0.25 | 25 |
Bahr el Ghazal | 0 | 435 | 446 | −3 | −10 | 1.00 | 1 |
Sudd | 35 | 162 | 201 | −9 | −6 | −1.25 | 12 |
Baro-Akobo-Sobat | 0 | 242 | 232 | −1 | 0 | −1.17 | 13 |
Albert Nile-Bahr al Jabal | 33 | 90 | 91 | −2 | −1 | −0.08 | 35 |
Victoria Nile | 28 | 100 | 93 | 3 | 5 | −0.56 | 28 |
Semliki-L.Albert | 0 | 78 | 72 | 0 | 0 | 0.43 | 5 |
Lake Victoria | 5 | 246 | 208 | 6 | 8 | 2.00 | 28 |
Kagera | 0 | 57 | 52 | 0 | 0 | 0.78 | 5 |
| | | | | | | |
NILE 2005 to 2010 | | 2013 | 1987 | 5 | 2 | 3 | 14 |
The water balance check shows that P, ET and ΔS spatial data from different types of satellites are consistent with the classical hydrological observations. The minor differences can be ascribed to ungauged lateral transfer of water outside the main river courses, apart from the fact that the reporting period of the discharge stations and the satellite data are not identical. Discharge measurement stations are not always located at the boundary between adjoining basins. For instance, the peak flow in Dongola is generally known to be 84 km3/yr, but Dongola is located in the central part of the Main Nile 3 sub-basin. The peak flow computed from P-ET is 87 km3/yr and occurs upstream of the Merowe dam.
Figure 6.
Relationship between estimated outflow through the main outlet of each sub-basin for the period 2005–2010 and the longer term averaged measured river discharge.
Figure 6.
Relationship between estimated outflow through the main outlet of each sub-basin for the period 2005–2010 and the longer term averaged measured river discharge.
Considering this good agreement with flows and the plausibility of non-conventional inter-basin transfers, which cannot be neglected, it can be concluded that the P-ET values are highly realistic.
Figure 5 is, to the authors’ knowledge, the first map of water producing and water consuming areas published for the Nile basin, and due to the acceptable values for inter-basin border flow, this information can be used for water accounting. Some examples of estimated drainage and withdrawals by agro-ecosystems are discussed in the next section.
3.4. Net Water Producers and Consumers
The FAO-Africover Land Use-Land Cover (LULC) product was used as a basis for identifying the agro-ecosystems with net production and net consumption of water. The results of the LULC map and the link to rainfall and ET are provided in
Table 5. The complete data set for 43 LULC classes is provided in
Appendix 1. The net water production comprises all non-consumed and non-utilized water resources and is defined simply as P-ET. It is the total drainage of all non-consumed water, assuming zero changes in storage. In cases where ET > P prevails, an effective rainfall coefficient of 70% (ω = 0.7) has been used arbitrarily, hence the surface runoff and drainage production is 30% of P. In reality, the ω-factor is climate, soil and land use dependent (e.g., Dastane, [
76]), and more research is required to improve the estimation of ω across heterogeneous landscapes. The remaining ET that cannot be met from P is referred to as the incremental ET due to surface water or groundwater withdrawals. This extra water consumption is feasible due to lateral transfer of water in the basin and can happen by manmade infrastructure or through natural groundwater flows.
This analysis reveals that plantations are, in a relative sense, the major sources of exploited water in the River Nile (P-ET: 300 mm/yr). Rainfed crops follow with a water layer of P-ET being 181 mm/yr. Plantations and rainfed crops are both Modified Land Use. It is a positive signal that these forms of land use generated by humans are the source of lateral flow. Because the rainfed crop area is five times larger than for forest plantations, crop plantations generate 68.7 km
3/yr excess water (runoff, drainage, recharge), in addition to providing staple food. This is an excellent example of contributing simultaneous to food security and ecosystem services (Rebelo and McCartney, 2012 [
77]). The majority of these plantations can be found in the equatorial Nile region.
Table 5.
Water production and consumption by land use-land cover class in the entire Nile basin for the period 2005–2010. Production was computed as P-ET when P > ET. For cases with ET > P, production was computed as 0.3 P. Net withdrawal was computed as ET-0.7 P.
Table 5.
Water production and consumption by land use-land cover class in the entire Nile basin for the period 2005–2010. Production was computed as P-ET when P > ET. For cases with ET > P, production was computed as 0.3 P. Net withdrawal was computed as ET-0.7 P.
WA+ Code | LULC | Area | ET | ET | P | P | P-ET | Production | Net Withdrawals |
---|
| | (km2) | (mm/yr) | (km3/yr) | (mm/yr) | (km3/yr) | (mm/yr) | (km3/yr) | (km3/yr) |
---|
MLU2 | Rainfed crops | 380,180 | 749 | 284.6 | 929 | 353.3 | 181 | 68.7 | 0 |
MLU1 | Plantations | 74,806 | 850 | 63.6 | 1150 | 86.0 | 300 | 22.4 | 0 |
ULU4 | Savannah | 892,666 | 846 | 755.3 | 870 | 776.2 | 23 | 20.9 | 0 |
ULU8 | Pastures | 441,240 | 423 | 186.9 | 442 | 194.9 | 18 | 8.1 | 0 |
MWU17 | Wetlands | 10,057 | 1040 | 10.5 | 1092 | 11.0 | 52 | 0.5 | 0 |
MWU11 | Urban areas | 58 | 769 | 0.0 | 815 | 0.0 | 46 | 0.0 | 0 |
ULU19 | Sinks | 987 | 297 | 0.3 | 146 | 0.1 | −150 | 0.0 | −0,2 |
MLU12 | Urban areas | 4740 | 473 | 2.2 | 142 | 0.7 | −330 | 0.2 | −1,8 |
PLU4 | Bare land | 196,017 | 91 | 17.9 | 64 | 12.6 | −27 | 3.8 | −9,1 |
MWU6 | Reservoirs | 6310 | 1566 | 9.9 | 48 | 0.3 | −1518 | 0.1 | −9,7 |
ULU11 | Bare land | 679,835 | 62 | 42.1 | 52 | 35.4 | −10 | 10.6 | −17,3 |
MWU1 | Irrigated crops | 54,733 | 812 | 44.5 | 282 | 15.5 | −530 | 4.6 | −33,6 |
ULU16 | Rivers and natural lakes | 89,489 | 1445 | 129.3 | 1335 | 119.4 | −110 | 35.8 | −45,7 |
ULU10 | Wetlands | 112,648 | 1206 | 135.9 | 960 | 108.1 | −247 | 32.4 | −60,2 |
ULU1 | Forests | 285,271 | 1067 | 304.4 | 1053 | 300.5 | −14 | 90.2 | −94,1 |
Total | | 3,229,039 | | 1987.3 | | 2014.1 | | 298 | −272 |
A layer of 1581 and 530 mm/yr, respectively, is evaporated in excess of rainfall with respect to reservoirs and irrigated crops. As pointed out before, the source of this incremental ET is not only surface water, but can also be groundwater that seeps out in flood plain areas and in shallow water table areas. The largest volumetric water consumer is forest. In addition to rainfall, forests consume 94 km
3/yr extra, most likely due to deep rooting and use of groundwater interflows. Forests in the Nile must be distinct groundwater-dependent ecosystems. The total forest ET is 304.4 km
3/yr, indicating that forest in the Nile basin are substantial water consumers (304/1987 × 100%=15%). The gross rainfall is 300.5 km
3/yr, and all this rainfall water is certainly not available for forest ET, especially during heavy storms when surface runoff from forested mountains is large. There is no runoff from forests during the dry season, and the fact that ET remains high during the dry season implies that groundwater must be tapped into. The ω = 0.7 correction factor is used for forests also. More research is needed on local forest hydrology processes to estimate which part of the rainfall is stored in the root zone and subsequently available for root uptake during dry years, and which part of the rainfall runs off. Van Eekelen
et al. ([
40]) found for the Incomati basin that the net withdrawals by forests were also substantially higher than irrigated crops, which supports the findings for the Nile basin. Wetlands in the Nile basin have a net withdrawal of −60.2 km
3/yr, which reduces the flow of water to downstream areas. The incremental ET from open water bodies (
i.e., rivers and natural lakes; see
Appendix 1) totals 45.7 km
3/yr (
Table 5). This total ET value for natural bodies of water should not be confused with the reservoirs that have an incremental ET of 9.7 km
3/yr. Hence, the natural lakes and rivers consume more water than the manmade lakes.
The irrigated area of 54,733 km
2 is in agreement with earlier estimates of 55,360 km
2 and 49,010 km
2 made by Awulachew
et al. [
78] and Bastiaanssen and Perry [
79], respectively. FAO [
80] estimated the total irrigation-equipped area in the basin at 50,790 km
2. Hence, there are some acceptable differences that can be attributed to the type of data survey, the period under consideration, and definitions of irrigated areas.
Table 5 indicates that the net withdrawal by the irrigation sector is 33.6 km
3/yr. Bastiaanssen and Perry [
79] estimated the total net withdrawal to be 31.2 km
3/yr for the year 2007; their value is remarkably similar to the value derived in this study, given that different data sources were used, and 2007 was a wet year with lower irrigation water requirements.
Awulachew
et al. [
78] estimated the current annual irrigation requirement—or gross withdrawal—to be 65.9 km
3/yr, which implies an average irrigation efficiency of 50% at a net withdrawal of 33.6 km
3/yr, which is a reasonable number for the current flood irrigation practices in the Nile basin. Hilhorst
et al. [
4] report a basin-wide average irrigation efficiency of 49%. The FAO estimates of gross withdrawal were 67.7 km
3/yr in 1997 and are thus close to Awulachew
et al. [
78].