Hydrology analysis

We use the Variable Infiltration Capacity (VIC) hydrologic simulation model (Liang et al. 1994) to explore the potential hydrologic impacts of the CHIRPS rainfall trends. The VIC model, which is described in detail in Njissen et al. (1997, 2001a, 2001b), has been used to study climate impacts to large river basins in the US and around the world (e.g. Nijssen et al. 2001c). It is currently used for monitoring water budgets and drought in Africa by the Land Surface Hydrology Group at Princeton University. In our experiment VIC iss forced with a daily version of CHIRPS that is spatially aggregated to 0.25° latitude x 0.25 ° longitude resolution. Other atmospheric forcings for the model come from the Princeton University dataset (Table 1). We examine changes to simulated annual evapotranspiration and surface runoff. Daily data for these variables were summed to the month then averaged over the October to September water year. The VIC model calculates the moisture fluxes for each grid cell independently. Evapotranspiration is from canopy evaporation, evaporation from bare soils, and transpiration (Liang et al, 1994). VIC uses a standalone routing model that is based on a representation of reservoirs and streams (Njissen et al., 2001c). Trends in runoff at each grid cell would occur from changes to local and upstream runoff changes. Trends in runoff do not account for influence of groundwater-surface water interactions on stream flow because this is not modeled in VIC (Trambauer et al., 2013). Evapotranspiration and runoff trends were calculated at each grid cell using the Theil-Sen method and statistical significance was identified using Kendall’s tau (p < 0.05). The model was initialized on

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January 1982. Several years of outputs were omitted from the analysis to account for model spin up. Trends were calculated for the 1984 to 2009 period.

To identify where changes in surface runoff could be important for water resources we overlay a map of the major water basins in Tanzania on the trend results. We created this shapefile by combining two sources. The base map was the USGS HydroSHEDS Drainage Basin (Beta) 30 second version which was downloaded from

http://hydrosheds.cr.usgs.gov/dataavail.php. This map has many small scale basins, so we used ArcMap version 10.1 to aggregate them into nine major water basins. These are the major basins according to Tanzania’s Ministry of Water. They are described in Table 2. Detailed descriptions of the basins and the map used for aggregation of HydroSHEDS data can be found at http://maji.go.tz/basins/index.php. The shapefile for river outlines came from the USGS Hydro1K dataset and was downloaded at

http://www.fao.org/geonetwork/srv/en/metadata.show?id=30934&currTab=simple. The shapefile of major lakes was from the Global Lakes and Wetlands Database (GLWD- Level1) and was downloaded at http://www.wwfus.org/science/data.cfm.

We conduct an additional VIC experiment to test if changes in temperature are contributing to the evapotranspiration and runoff trends. Princeton data show significant increases in annual mean air temperature between 0.1 °C and 0.5 °C per decade in Tanzania (not shown), which could influence evapotranspiration and runoff by increasing evaporative demand. We remove any long term trends that are present in the Princeton temperature data using the method from Hamlet et al., 2005. In the de-trended data the mean of each month is equal to the climatological mean but daily variability remains the same as in the original data. The

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VIC model is run again using the de-trended temperature data. Trends are recalculated using evapotranspiration and runoff from this experiment.

Table 2. Tanzania’s nine major water basins Text from http://maji.go.tz/basins/nine.php ID Name Basin description 1 Lake Victoria

Basin

A source of the White Nile River 2 Internal Drainage

Basin

Internal drainage basins of Lake Eyasi, Manyara and Bubu Complex. Rainfall varies greatly from year to year.

3 Pangani Basin Major water uses are domestic, irrigation, industries, hydropower, livestock, fisheries, recreation, transportation

4 Wami/Ruvu Basin

Basin consists of wide plains and large mountain ranges and consists of two main river systems, the Wami and the Ruvu, and coastal rivers south of Dar es Salaam

5 Rufiji Basin The climate in the basin differs from the coast (two rainy seasons) to the highlands in the upper part of the catchment (unimodal rainfall). Land use in the Basin is agriculture, mining, forestry, livestock keeping, fishing, wild life, navigation, and human settlements.

6 Ruvuma River and Southern Coast Basin

Basin is comprised of five major independent rivers that drain into the Indian Ocean. Ruvuma River is shared by Tanzania and Mozambique.

7 Lake Nyasa Basin

Lake Nyasa is shared by Tanzania, Malawi, and Mozambique. The Tanzania side of the basin has a drainage area of 37,000 km2 (28% of the total drainage area in the lake).

8 Lake Rukwa Basin

Major water uses are domestic water supply for urban centers Mbeya and Sumbawanga and for smaller towns and rural areas. Also for smallholder irrigation. Important source in the dry season despite poor water quality.

9 Lake Tanganyika Basin

The Tanzania side of the basin contributes 60% of the total runoff to Lake Tanganyika. The main river is the Malagarasi. Its main tributaries are Moyowosi River from the north and Igome River from the east.