**3. Results**

#### *3.1. General Trends in EVI, Soil Moisture, TWSA, and Precipitation*

Broadly speaking, there was a dominant positive trend in vegetation greenness anomalies in most parts of Africa (Figure 2) over the study period. Nonetheless, large clusters of a negative trend in greenness anomalies can be observed in Algeria, Tunisia, Libya, Niger, Nigeria, Ghana, Angola, and in countries in the eastern flank of Africa, extending from Eretria southwards to South Africa, and eastwards to Madagascar. Interestingly, the trajectories of vegetation greenness anomalies spatially coincided with trends in one or more of the hydrological variables in some locations. For example, the positive trend in greenness anomalies in southern Mali, in the region around the Sudan/South Sudanese border, and in parts of Angola and South Africa, is matched by an upward trend in at least two of the hydrological variables. However, a diverging direction of anomalies trends between vegetation and particularly soil moisture and TWSA, can be observed in Libya, Egypt, and also in Zambia. Finally, an important trend of ecological note is the pronounced clusters of a negative trend in precipitation and TWSA anomalies in parts of the Congo Basin.

**Figure 2.** Trends in anomalies of EVI, precipitation, soil moisture, and TWSA over Africa from 2003 to 2015. The white areas are either no trend (*p* > 0.05), barren, no data, or water bodies.

#### *3.2. Relationship between Vegetation Greenness Dynamics and Water Availability*

Figure 3 shows the maps indicating the strength of the relationship between vegetation greenness and the hydrological variables and their lags as modeled with values of original observations (Figure 3A) and those of the monthly anomalies (Figure 3B). In the case of the original values, the relationship was very strong for most of Africa (LCCC > 0.75), although moderate relationships (LCCC = 0.5–0.75) were observed in Somalia, parts of Ethiopia and Kenya, southern Namibia, and Western South Africa, and in the northern margins of the Sahel. On the other hand, anomalies in vegetation greenness were generally less coupled to anomalies in water availability in many parts of Africa (LCCC < 0.25), notably in areas quite north of the Equator, predominantly across the savanna zones from west Senegal to the east coast of Eretria and Djibouti. However, in Namibia, Botswana, Kenya, Somalia and parts

of Tanzania, Ethiopia, and South Africa, a moderate relationship between anomalies in vegetation greenness and water availability was evident (LCCC = 0.5–0.75).

**Figure 3.** Spatial variability of the strength (LCCC) of vegetation greenness response to concurrent and lagged precipitation, soil moisture, and TWSA for the period 2003 to 2015 based on monthly time series of (**A**) original observations and (**B**) anomalies. The white areas are masked out either due to no data, barren land, or water bodies.

Figure 4 summarizes the relationship between vegetation greenness and the hydrological variables and their lags across the major land cover types. Again, the relationship was stronger for all land cover types in the case of original values (Figure 4A) in comparison to the anomalies (Figure 4B). Furthermore, relative to the other land cover types, the relationship was generally stronger in the woodlands and croplands with the original values, whereas vegetation greenness anomalies were more coupled with anomalies in water availability in the grasslands than in the other land cover types. Regardless of which data type (original or anomaly values) was used, vegetation greenness in the forest class was relatively less associated with water availability than in the other land cover types (Figure 4A,B).

**Figure 4.** Relationship between vegetation greenness and concurrent and lagged precipitation, soil moisture, and TWSA across major land cover types for the period 2003 to 2015 based on monthly time series of (**A**) original observations and (**B**) anomalies.

*3.3. Relative Importance of Soil Moisture, Precipitation, and TWSA as Drivers of Vegetation Greenness Dynamics*

Figure 5 shows the two top-ranked predictor variables and their associated lags from the modeled relationship between vegetation greenness and water availability for the original (Figure 5A) and monthly anomalies (Figure 5B) data. Figure 6 further illustrates the proportion of pixels across the five top-ranked predictors. Based on the original observations, precipitation and soil moisture were the most important predictors of vegetation greenness. The proportion of pixels where soil moisture was ranked as the topmost VIP in predicting vegetation greenness was about 55% (Figure 6A). This encompassed parts of west southern Africa, East Africa, and the western Maghreb (Figure 5A). However, the proportion rapidly declined to about 30% in the fifth VIP. Precipitation, on the other hand, was the most important hydrological variable driving vegetation greenness in the eastern flank of southern Africa (Figure 5A). The proportion of pixels across Africa where precipitation was ranked among the top five VIP was fairly constant (45%, Figure 6A). In terms of vegetation lag response to the hydrological variables, and based on the first two important predictors, precipitation generally led vegetation greenness by 1 to 2 months (e.g., in eastern South Africa), whereas soil moisture was more concurrent with or led vegetation greenness by 1 month (e.g., in Namibia and Botswana).

**Figure 5.** Spatial distribution of the top two ranked VIPs of the hydrological variables and their associated lags in (months) in modeling vegetation response to water availability in Africa based on monthly time series of (**A**) original observations and (**B**) anomalies. The white areas are masked out either due to no data, barren land, or water bodies.

**Figure 6.** Summary of the first five ranked variable of importance of the hydrological variables and their lags in driving vegetation response to water availability across Africa based on (**A**,**B**) original and (**C**,**D**) anomalies monthly data.

Anomalies in soil moisture and TWSA were the dominant hydrological predictors of anomalies in vegetation greenness in most of Africa, with the role of precipitation greatly diminished (Figure 6C). Whereas soil moisture anomalies drive anomalies in vegetation greenness in virtually all parts of Africa, anomalies in TWSA were more coupled to vegetation dynamics anomalies in the humid savanna zone of Nigeria and the semi-arid region of Egypt (Figure 5B). Nonetheless, the proportion of pixels with dominant soil moisture control of vegetation greenness anomalies decreased considerably from the first (65%) to the fifth (30%) ranked VIP. This is in contrast to that of TWSA that increased from 25% to 50% (Figure 6D). Going by the first and second VIP results, vegetation anomalies lagged precipitation and TWSA anomalies by 0 to 1 month, though the response was much delayed in some parts of North Africa (Figure 5B).
