Rainfall Consistently Enhanced Around the Gezira Scheme in East Africa

SUPPLEMENTARY INFORMATION DOI: 10.1038/NGEO2514

Rainfall consistently enhanced around the Gezira Scheme in EastSupplementary Africa Information due to irrigation Ross E. Alter, Eun-Soon Im, Elfatih A. B. Eltahir

Mechanistic Framework

Two sets of theoretical pathways have been discussed in previous studies connecting irrigation to

rainfall modification – a dynamical framework and a moisture budget framework. Both

frameworks begin with increased soil moisture and plant growth due to irrigation, which create a

repartitioning of the surface energy budget, i.e., smaller sensible heat flux and greater latent heat

flux. This results in lower surface air temperatures and greater evapotranspiration, which

increase the specific humidity and relative humidity of the air. Then the two frameworks

diverge. The dynamical pathway (Supplementary Fig. 6) assumes that the air becomes more

convectively stable, the height of the planetary boundary layer is lowered, and subsidence occurs

over the irrigated area. This subsidence forms an area of anomalous high pressure, which

decreases the potential for convective storms, and thus rainfall, over the irrigated area. However,

this area of higher pressure also causes an anomalous anti-cyclonic circulation emanating from

the irrigated area. The clockwise wind anomalies from the anti-cyclonic circulation would then

converge with the southwesterly background winds to the south and east of the irrigated area and

cause anomalous upward vertical motion, which could serve as a trigger and catalyst for storm

development1. This mechanism would thus support enhanced rainfall relatively close to (within

a few hundred kilometers of) the irrigated area.

The above mechanism may offer a hint as to why surface air temperature in Gedaref experienced

a greater temperature decrease than irrigated Gezira, according to the NCEP-NCAR reanalysis.

Over Gezira, the increase in soil moisture due to irrigation causes a subsequent decrease in land

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© 2015 Macmillan Publishers Limited. All rights reserved surface temperature (see Fig. 1). However, the irrigation-induced development of anomalous high pressure may reduce the amount of cloud cover over Gezira and allow more radiation to penetrate to the surface, potentially causing a negative feedback on the original decrease in surface air temperature. However, horizontal convergence and subsequent ascent in Gedaref may cause an increase in cloud cover, which prevents further radiation from reaching the surface.

Combined with the cooling effect from enhanced rainfall wetting the soil (akin to irrigation), temperatures would be expected to cool considerably in Gedaref. Thus, the cooler temperatures in Gedaref in comparison to Gezira may be consistent with the physical mechanism described above.

Another potential mechanism for irrigation-induced rainfall involves the moisture budget2,3. The large increase in evapotranspiration over the irrigated area would create a surplus of atmospheric water vapor and thus increase convective available potential energy (CAPE) over the irrigated area. To a lesser degree, CAPE would also be enhanced downwind of the irrigated areas since excess moisture would likely be advected by the background wind into these non-irrigated regions. Over the irrigated areas themselves, the decrease in surface air temperature could increase convective inhibition (CIN)1, which would reduce the frequency of convective rainfall events over the irrigated area. Downwind, the increase in CAPE and little change in CIN would create an environment that is more conducive for convective storm development. Even if large- scale, synoptic processes are the dominant control on the frequency (i.e., triggering) of rainfall events in these downwind regions, moisture transport and convergence downwind could still feed more water into existing storms, thereby increasing the intensity of rainfall events. This mechanism would theoretically result in irrigation-induced rainfall several hundred kilometers

© 2015 Macmillan Publishers Limited. All rights reserved from the irrigation source. On smaller spatial scales, excess moisture over the irrigated area could also be absorbed by a passing frontal system and potentially intensify rainfall locally4.

References

1. Im, E.-S., Marcella, M. P. & Eltahir, E.A.B. Impact of potential large-scale irrigation on

the West African monsoon and its dependence on location of

irrigated area. J. Clim. 27, 994–1009 (2014).

2. DeAngelis, A., et al. Evidence of enhanced precipitation due to irrigation over the Great

Plains of the United States. J. Geophys. Res. 115, D15115, doi:10.1029/2010JD013892

(2010).

3. Alter, R. E., Fan, Y., Lintner, B. R., & Weaver, C. P. Observational evidence that Great

Plains irrigation has enhanced summer precipitation intensity and totals in the

Midwestern US. J. Hydrometeorol., in press (2015).

4. Barnston, A. G. & Schickedanz, P. T. The effect of irrigation on warm season

precipitation in the Southern Great Plains. J. Clim. Appl. Meteorol. 23, 865–888 (1984).

(b) Elevation (m)

© 2015 Macmillan Publishers Limited. All rights reserved Supplementary Fig. 1: (a) A map of major irrigated (green) and rainfed (pink) agricultural schemes in Sudan, with a larger Sudan reference domain in the bottom right corner. Note the locations of Wad Medani (irrigated, analyzed in Fig. 3), Gedaref (rainfed, analyzed in Figs. 3 and 4), and (Northern) Kordofan (minimal agriculture, analyzed in Fig. 4). (b) Land elevation (m) inside the model domain. The Gezira Scheme is encompassed by a black rectangle in both panels. The top figure has been altered from the original figure from UNEP / GRID Europe, which was sourced from SIM (Sudan Interagency Mapping); FAO; vmaplv0, gns, NIMA; The Gateway to Astronaut Photography of Earth, NASA; various reports, maps and atlases; UN Cartographic Section.

© 2015 Macmillan Publishers Limited. All rights reserved Control - Mean Rainfall Observed (a) (c) © 2015 Macmillan Publishers Limited. All rights reserved (d) (b) Supplementary Fig. 2: (top) Differences in mean rainfall (mm day-1) between the control (CONT) simulation of the model and UDel gridded observations for 1979-2008 during (a) July and (b) August. (bottom) Mean July-September rainfall (mm day-1) for (c) CONT and (d) UDel over 1979- 2008. The Gezira Scheme is encompassed by a black rectangle in all panels.

© 2015 Macmillan Publishers Limited. All rights reserved Observed Simulated (c) (a) July © 2015 Macmillan Publishers Limited. All rights reserved (b) (d) August Supplementary Fig. 3: Same as Figure 2, except for two differences: 1) The shaded grid cells are percent differences (%) in rainfall, and 2) the center of the “observed” scale is shifted below zero to account for the general decrease in rainfall over the Gezira region.

(b)

(c)

Wad Medani Gedaref

GHCN spatial and temporal trends for AugustDifference in Normalized Aug Rainfall

(b)

(c)

© 2015 Macmillan Publishers Limited. All rights reserved Supplementary Fig. 5: (a) A map of different tributaries of the Nile River and their watersheds (http://entroportal.nilebasin.org/OSIKit/Lists/Basin%20Definiti on/DispForm.aspx?ID=2). The black rectangle, denoting the Gezira Scheme, was added by the authors. The Blue Nile extends southeast of the Gezira Scheme, the White Nile extends southwest of the Gezira Scheme, the Main Nile extends north of the Gezira Scheme, and the Atbara River lies to the east of the Gezira Scheme (shaded in dark brown). (middle and bottom) Differences in normalized streamflow discharges during (b) July and (c) August. Comparisons between 1965-74 and 1975-89 (blue) and 1965-74 and 1975-99 (yellow) are shown for streamflow gauges on four major Nile tributaries: Blue Nile (Deim), White Nile (Malakal), Main Nile (Hasanab), and Atbara River (Kubur).

© 2015 Macmillan Publishers Limited. All rights reserved © 2015 Macmillan Publishers Limited. All rights reserved Supplementary Fig. 6: A mechanistic pathway from irrigation to rainfall modification. The diagram on the left shows how irrigation may lead to local (red outline) and remote (blue outline) effects on rainfall in the model experiments (reference 14). The colored arrows point to corresponding panels of simulated, irrigation-induced absolute differences in: (top left) July 2-m air temperature (K), (bottom left) July omega (Pa s-1; negative values correspond to upward vertical motion), and (top right and bottom right) July rainfall (mm day-1; from Fig. 2).