Analyses of the relationship between drought occurrences and their causal factors in Tigray Region, Northern Ethiopia

Figures & data

Fig. 1. Map of the study area.

Fig. 2. Mean monthly (a) and mean annual (b) precipitation and temperature of the study area for the period 1901 to 2016.

Fig. 3. Temporal pattern of SPEI at 1-month, 3-month, 6-month, 12-month and 24-month time scales (1982 to 2016).

Table 1. Test of randomness and the product-moment correlation coefficient values representing the trend of drought conditions in Tigray region at 1-month, 3-month, 6-month, 12-month and 24-month time scale (1982–2016).

Time scale	Run’s test (z)	Correlation coefficient (r)	p-value
1-month	−4.01	−0.274**	0.000
3-month	−12.21	−0.330**	0.000
6-month	−15.24	−0.335**	0.000
12-month	−17.29	−0.367**	0.000
24-month	−19.25	−0.472**	0.000

^** Correlation is significant at the 0.01 level (2-tailed).

Fig. 4. Map of sampled tropical Indian Ocean, tropical Atlantic Ocean, tropical Pacific Ocean, the Red Sea and Nino 3.4 regions (12-month average global SSTAs for the period 1982 to 2016).

Table 2. Trend test summary of the global and local factors used in drought causation analyses (z = run’s test, r = Pearson’s correlation coefficient, p = probability-value) for the period 1982 to 2016.

Climate control factors	1-month			3-month			6-month			12-month			24-month
	z	r	p	z	r	p	z	r	p	z	r	p	z	r	p
Tropical Indian Ocean (SSTA)	−13.48	0.63**	0.000	−16.55	0.69**	0.000	−18.53	0.74**	0.000	−18.86	0.79**	0.000	−19.25	0.86**	0.000
Tropical Atlantic Ocean (SSTA)	−13.97	0.613**	0.000	−16.45	0.651**	0.000	−17.74	0.692**	0.000	−18.66	.747**	0.000	−18.95	0.802**	0.000
Tropical Pacific Ocean (SSTA)	−14.27	0.49**	0.000	−15.57	0.53**	0.000	−17.64	0.58**	0.000	−18.66	0.61**	0.000	−18.54	0.67**	0.000
Nino 3.4 (SSTA)	−17.88	−0.07	0.17	−18.41	−0.07	0.18	−18.23	−0.06**	0.19	−18.76	−0.06	0.22	−18.64	0.053	0.000
Red Sea (SSTA)	−9.38	0.54**	0.000	−14.50	0.63**	0.000	−16.07	0.69**	0.000	−18.27	0.76**	0.000	−18.24	0.80**	0.000
SOI	−7.91	0.18**	0.000	−14.40	0.21**	0.000	−15.97	0.23**	0.000	−17.67	0.26**	0.000	−18.44	0.33**	0.000
NDVI	−11.22	0.59**	0.000	−15.74	0.66**	0.000	−19.51	0.76**	0.000	−19.85	0.88**	0.000	−19.75	0.89**	0.000
Albedo	−19.25	−0.75**	0.000	−19.20	−0.78**	0.000	−19.71	−0.81**	0.000	−19.75	−0.85**	0.000	−19.75	−0.87**	0.000

^** Correlation is significant at the 0.01 level (2-tailed).

Fig. 5. Running means of the sea surface temperature anomalies for the (a) tropical Indian Ocean, (b) tropical Atlantic Ocean, (c) tropical Pacific Ocean and (d) Red Sea region on 1-month, 3-month, 6-month, 12-month and 24-month basis.

Fig. 6. Running means of the (a) Nino 3.4 region surface temperature anomaly and (b) Southern Oscillation Index (SOI) on 1-month, 3-month, 6-month, 12-month and 24-month basis.

Fig. 7. Temporal pattern of average NDVI and Albedo at 1-month, 3-month, 6-month, 12-month and 24-month time scales.

Table 3. Correlation coefficient (r) values between zero-lag SPEI and the local and global climate controlling factors at 1-month, 3-month, 6-month, 12-month and 24-month time scales for the period 1982 to 2016.

Climate control factors	1-month	p-value	3-month	p-value	6-month	p-value	12-month	p-value	24-month	p-value
Tropical Indian Ocean (SSTA)	−0.242**	0.000	−0.280**	0.000	−0.282**	0.000	−0.340**	0.000	−0.490**	0.000
Tropical Atlantic Ocean (SSTA)	−0.208**	0.000	−0.269**	0.000	−0.247**	0.000	−0.302**	0.000	−0.413**	0.000
Tropical Pacific Ocean (SSTA)	−0.153**	0.002	−0.178**	0.000	−0.200**	0.000	−0.268**	0.000	−0.328**	0.000
Red Sea (SSTA)	−0.140**	0.004	−0.172**	0.000	−0.135**	0.006	−0.133**	0.007	−0.207**	0.000
Nino 3.4 (SSTA)	0.021	0.672	−0.016	0.739	−0.100*	0.043	−0.148**	0.003	−0.076	0.129
SOI	−0.115*	0.018	−0.143**	0.003	−0.098*	0.046	−0.041	0.413	−0.165**	0.001
NDVI	−0.196**	0.000	−0.244**	0.000	−0.263**	0.000	−0.283**	0.000	−0.374**	0.000
Albedo	0.117*	0.017	0.179**	0.000	0.208**	0.000	0.287**	0.000	0.390**	0.000

^* Correlation is significant at the 0.05 level (2-tailed).

^** Correlation is significant at the 0.01 level (2-tailed).

Fig. 8. Scatter plots showing the zero-time-lag observed SPEI against predicted SPEI values at (a)1-month, (b) 3-month, (c) 6-month, (d) 12-month and (e) 24-month time scales.

Table 4. Correlation coefficient (r) values between time-lagged SPEI and the local and global climate controlling factors at 1-month, 3-month, 6-month, 12-month and 24-month time scales for the period 1982 to 2016.

Climate control factors	1-month	p-value	3-month	p-value	6-month	p-value	12-month	p-value	24-month	p-value
Tropical Indian Ocean (SSTA)	−0.240**	0.000	−0.303**	0.000	−0.286**	0.000	−0.344**	0.000	−0.486**	0.000
Tropical Atlantic Ocean (SSTA)	−0.221**	0.000	−0.253**	0.000	−0.248**	0.000	−0.300**	0.000	−0.403**	0.000
Tropical Pacific Ocean (SSTA)	−0.171**	0.000	−0.217**	0.000	−0.219**	0.000	−0.278**	0.000	−0.332**	0.000
Red Sea (SSTA)	−0.137**	0.005	−0.153**	0.002	−0.142**	0.004	−0.136**	0.006	−0.202**	0.000
Nino 3.4 (SSTA)	0.030	0.538	0.001	0.982	−0.097*	0.048	−0.166**	0.001	−0.087	0.082
SOI	−0.111*	0.024	−0.139**	0.004	−0.073	0.136	−0.021	0.667	−0.149**	0.003
NDVI	−0.148**	0.003	−0.198**	0.000	−0.216**	0.000	−0.273**	0.000	−0.365**	0.000
Albedo	0.143**	0.003	0.175**	0.000	0.206**	0.000	0.281**	0.000	0.381**	0.000

^* Correlation is significant at the 0.05 level (2-tailed).

^** Correlation is significant at the 0.01 level (2-tailed).

Fig. 9. Scatter plots showing the time-lagged observed SPEI against predicted SPEI values at (a)1-month, (b) 3-month, (c) 6-month, (d) 12-month, (e) 24-month time scales.

Availability of data and materials

The climate data used in this study are available from the corresponding author upon request.