Countries: Ethiopia, Kenya and Tanzania

Introduction

Wheat is the second-most-produced cereal grain behind maize in the world (FAOSTAT, 2022). The global production volume of wheat came to about over 761 Mton in the marketing year of 2019/20 (FAOSTAT, 2022). Cereals like maize, rice and sorghum, millet and wheat are major staple foods for most people in Africa. Africa produces around 25 Mton of wheat on 10 Mha (FAOSTAT, 2022). The wheat area s makes around 8% of the area used for cereal production in Africa (FAOSTAT, 2022). Around 19 percent of all wheat lands in Africa are located in the three sub-Saharan countries (Table 1).

Table 1. Annual wheat harvest area in sub-Saharan Africa
Country	Annual wheat harvest area (1000 ha)*	Percentage of total  wheat lands in Africa
Ethiopia	1737.3	17.5
Kenya	134.9	1.4
Tanzania	58.5	0.6
Total	1930.7	19.4
* The data are from FAOSTAT for the period from 2015 to 2020

 

The crop model

The Python Crop Simulation Environment of WOFOST (WOrld FOod STudies) was used for the implementation (https://pcse.readthedocs.io/en/stable/). This model takes into account phenological development, leaf development, light interception, CO₂ assimilation, root growth, transpiration, respiration and partitioning of assimilates (de Wit et al., 2019). Daily weather data, crop parameters, soil parameters, and management data are needed to run the model and simulate water-limited potential yield (Yw). The source of different data to do the simulations for each country were presented in Table 2.

Table 2.Sources of data to simulate rainfed wheat potential yield and for calculating the yield gap. Note: GYGA CA = GYGA country agronomist
	Sowing window	Daily weather data	Cultivar thermal time requirment**	Soil data	Actual yield
	GYGA CA	Measured and Propagated data*			GYGA CA
	GYGA CA	Propagated data
	GYGA CA	Propagated data			GYGA CA
* Details are in Van Wart et al. (2015) ** Calculated based on the sowing, flowering and maturity timing information from the GYGA country agronomist using weather data and cardinal temperatures *** AFrica Soil Information Service (Leenaars J.G.B., 2018)

Reference weather stations

The simulation was performed for 25 weather stations in the three? countries (Table 3). These were identified as reference weather stations for rainfed wheat in SSA countries based on the GYGA protocol (https://www.yieldgap.org/web/guest/methods-overview). SPAM (Spatial Production Allocation Model; https://www.mapspam.info/) maps, together with expert knowledge from agronomists and experts from these countries, were used to identify the rainfed wheat harvested area  (You et al., 2009, 2014). Following van Bussel et al (2015), a total of 25?? buffer zones were selected for rainfed wheat in SSA countries (Table 3, Fig. 1)

Figure 1. The location of the reference weather stations for rainfed wheat in Sub-Saharan African countries

Weather data

Weather data used in simulation included daily solar radiation, maximum and minimum temperatures, precipitation, vapor pressure deficit, and wind speed. Weather data for selected weather stations were subjected to quality control measures to fill in missing data and identify and correct erroneous values. In the case of those stations with only few years of weather data (Table 3), long-term weather data were generated based on correlations between measured weather data and NASA-POWER maximum and minimum temperatures (Van Wart et al., 2015). In the case of solar radiation and rainfall, the data from NASA-POWER were used without any correction (NASA, 2022). In the case of buffers without measure weather data at all, named the virtual stations (Table 3), uncorrected NASA-POWER data were used for all meteorological variables needed for crop modeling.

Soil information

Soil data was obtained from the AFSIS^[1] (Leenaars et al., 2018). Soil moisture content at field capacity, soil moisture content at wilting point, not infiltrating fraction of rainfall, initial soil water, and maximum rootable depth of the soil were obtained as soil parameters for each weather station.

Management data

The simulations were done under water-limited conditions with no limitation for nutrients. So, the sowing date within the buffers of the weather stations was the main management input data needed to run the model. For this purpose, the common sowing windows of rainfed wheat for each buffer were retrieved through agronomists from the countries (Fig. 2). These sowing windows and an algorithm were used to estimate the sowing date for each year for each weather station. The algorithm calculates the amount of cumulative rainfall for seven consecutive days into the sowing window. The last day of this period resulting in more than 20 millimeter cumulative rainfall was considered as the sowing date. If there was no consecutive seven days with at least 20 millimeter rainfall into the sowing window, the last day of the sowing window was as the sowing date.

Crop parameters

The crop parameters consist of parameter names and the corresponding parameter values that are needed to parameterize the components of the crop simulation model. These are crop-specific values regarding phenology, assimilation, respiration, biomass partitioning, etc. The same crop parameters were used for the simulations in all the weather stations except the parameters for phenology. The phenological parameters including the thermal time requirement from emergence to flowering and from flowering to harvesting were calculated based on the observed data for sowing, emergence, flowering and harvesting using the cardinal temperature and weather data at each weather station (Table 3).

Figure 2. The sowing window to plant rainfed wheat at the weather stations in Sub-Saharan Africa countries; The name and information of each station are presented in Table 3.

Table 3. Rainfed wheat cropping system and management information and the thermal time requirement of the cultivars at each weather station
Country	Station name	Station ID*	Longitude	Latitude	Elev	Water regime	Cropping system	Cropping cycle	Sowing window		Thermal time requirement**
									Start	End	From emergence to flowering	From flowering to maturity
Adet	Ethiopia	5000000	37.48	11.27	2240	Rainfed	Single: wheat	1	15-Jun	30-Jul	1310	1160
Ambo	Ethiopia	5000001	37.835	8.96	2100	Rainfed	Single: wheat	1	15-Jun	15-Jul	1210	1080
Ayira	Ethiopia	5000007	35.33	9.06	1700	Rainfed	Single: wheat	1	15-Jun	15-Jul	1300	1150
Butajira	Ethiopia	5000010	38.22	8.08	3252	Rainfed	Single: wheat	1	15-Jun	15-Jul	1130	1000
Debrezeit	Ethiopia	5000014	39	8.72	1900	Rainfed	Single: wheat	1	15-Jun	14-Jul	1150	1020
Kulumsa	Ethiopia	5000025	39.15	8	2241	Rainfed	Single: wheat	1	15-Jun	15-Jul	1050	940
Mekele	Ethiopia	5000026	39.47	13.52	1970	Rainfed	Single: wheat	1	15-Jun	15-Jul	1080	950
Melkassa	Ethiopia	5000027	39.33	8.4	1550	Rainfed	Single: wheat	1	15-Jun	15-Jul	1210	1080
Sheno	Ethiopia	5000032	39.3	9.344	2848	Rainfed	Single: wheat	1	1-Jul	31-Jul	890	790
Woliso	Ethiopia	5000035	37.97	8.55	2060	Rainfed	Single: wheat	1	15-Jun	15-Jul	1300	1150
Eth_rfwt2	Ethiopia	5000135	38.83	7.17	2675	Rainfed	Single: wheat	1	15-Jun	15-Jul	1200	1060
Eth_rfwt3	Ethiopia	5000136	40.11	7.77	2475	Rainfed	Single: wheat	1	15-Jul	15-Aug	1540	1360
Kakamega	Kenya	9000003	34.46	0.17	1399	Rainfed	Single: wheat	1	15-Aug	15-Sep	1150	1050
Kericho	Kenya	9000005	35.16	-0.22	1356	Rainfed	Single: wheat	1	15-Aug	15-Sep	1150	1050
Kitale	Kenya	9000008	34.96	0.97	1850	Rainfed	Single: wheat	1	15-Aug	15-Sep	1150	1050
Meru	Kenya	9000011	37.39	0.5	1044	Rainfed	Single: wheat	1	15-Sep	15-Oct	1150	1050
Nakuru	Kenya	9000012	36.6	-0.16	2557	Rainfed	Single: wheat	1	15-Aug	15-Sep	1150	1050
Eldoret	Kenya	9000015	35.3	0.48	2120	Rainfed	Single: wheat	1	1-Aug	31-Aug	1150	1050
Arusha	Tanzania	11000000	36.63	-3.32	1488	Rainfed	Single: wheat	1	5-Jan	3-Feb	1300	1200
Kia	Tanzania	11000003	37.07	-3.43	896	Rainfed	Single: wheat	1	4-Feb	5-Mar	1300	1200
Tan_rfwt1	Tanzania	11000125	36.92	-4.37	1313	Rainfed	Single: wheat	1	6-Dec	30-Dec	1300	1200
Tan_rfwt3	Tanzania	11000127	36.06	-2.88	914	Rainfed	Single: wheat	1	26-Jan	24-Feb	1300	1200
Tan_rfwt4	Tanzania	11000128	35.32	-3.65	1395	Rainfed	Single: wheat	1	5-Jan	3-Feb	1300	1200
Tan_rfwt5	Tanzania	11000129	37.52	-5.03	1041	Rainfed	Single: wheat	1	5-Jan	3-Feb	1300	1200
Tan_rfwt6	Tanzania	11000130	34.65	-3.54	1405	Rainfed	Single: wheat	1	6-Dec	30-Dec	1300	1200
* If the last three number of a station ID is more or equal to 100, the station is as a virtual station ** The cardinal temperatures to calculate the thermal time for sorghum are 0◦C as the base temperature, 30◦C or more than 30◦C  as the optimum temperature.

 

 

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