Table 5 displays MK and Sen's Slope (SS) statistic tests for detecting the significance of the trends and magnitudes of slopes, respectively, for drought indices (SPI, RDI, and SPEI) for the period of 1980–2017 (38-years). The tests were considered as two-sided assumptions at 1 and 5% levels of statistical significance. According to this table, during the spring season, drought detected by RDI and SPEI at a 1% significant level for Butajira station and by SPI at a 5% significant level for both Butajira and Wulbareg stations indicated a significant decreasing trend (increasing trends in DS), with the rate of change ranging from −0.03 to −0.04/year, while the remaining stations showed nonsignificantly decreasing trends.
Trend test statistics (Z) based on the modified MK as well as trend magnitudes estimated by 
for the three drought indices (SPI, RDI, and SPEI) from 1980 to 2017
| Time | Test | Spring (February–May) | Summer (June–September) | 6 months (March–August) | ||||||
|---|---|---|---|---|---|---|---|---|---|---|
| SPI | RDI | SPEI | SPI | RDI | SPEI | SPI | RDI | SPEI | ||
| Kulumsa | Z | −1.01 | −1.36 | −0.70 | 0.45 | −0.10 | 0.03 | 0.13 | −0.68 | 0.28 |
 | −0.01 | −0.02 | −0.01 | 0.01 | 0.00 | 0.00 | 0.00 | −0.01 | 0.01 | |
| Ziway | Z | −0.88 | −0.65 | −1.94 | 1.28 | 0.63 | −0.20 | 0.35 | −0.33 | −0.65 |
 | −0.01 | −0.01 | −0.03 | 0.02 | 0.01 | 0.00 | 0.01 | −0.01 | −0.01 | |
| Butajira | Z | − 2.24* | − 2.77** | − 2.87** | 0.28 | −0.03 | 0.68 | −1.01 | −1.21 | −1.01 |
 | −0.04 | −0.04 | −0.04 | 0.00 | 0.00 | 0.01 | −0.01 | −0.02 | −0.01 | |
| Sagure | Z | −1.28 | −1.43 | −1.76 | 0.10 | 0.00 | −0.08 | −1.03 | −1.08 | −1.33 |
 | −0.02 | −0.02 | −0.03 | 0.00 | 0.00 | 0.00 | −0.01 | −0.01 | −0.02 | |
| Wulbareg | Z | − 2.09* | −1.63 | −1.68 | 0.35 | 0.78 | 0.70 | −1.43 | −1.06 | −1.08 |
 | −0.03 | −0.03 | −0.03 | 0.01 | 0.01 | 0.01 | −0.02 | −0.01 | −0.01 | |
| Hawassa | Z | 0.15 | −0.78 | −0.85 | 0.10 | −0.55 | −0.63 | 0.20 | −0.58 | −0.75 |
 | 0.00 | −0.01 | −0.02 | 0.00 | −0.01 | −0.01 | 0.00 | −0.01 | −0.01 | |
| Time | Test | Spring (February–May) | Summer (June–September) | 6 months (March–August) | ||||||
|---|---|---|---|---|---|---|---|---|---|---|
| SPI | RDI | SPEI | SPI | RDI | SPEI | SPI | RDI | SPEI | ||
| Kulumsa | Z | −1.01 | −1.36 | −0.70 | 0.45 | −0.10 | 0.03 | 0.13 | −0.68 | 0.28 |
| | −0.01 | −0.02 | −0.01 | 0.01 | 0.00 | 0.00 | 0.00 | −0.01 | 0.01 | |
| Ziway | Z | −0.88 | −0.65 | −1.94 | 1.28 | 0.63 | −0.20 | 0.35 | −0.33 | −0.65 |
| | −0.01 | −0.01 | −0.03 | 0.02 | 0.01 | 0.00 | 0.01 | −0.01 | −0.01 | |
| Butajira | Z | − 2.24* | − 2.77** | − 2.87** | 0.28 | −0.03 | 0.68 | −1.01 | −1.21 | −1.01 |
| | −0.04 | −0.04 | −0.04 | 0.00 | 0.00 | 0.01 | −0.01 | −0.02 | −0.01 | |
| Sagure | Z | −1.28 | −1.43 | −1.76 | 0.10 | 0.00 | −0.08 | −1.03 | −1.08 | −1.33 |
| | −0.02 | −0.02 | −0.03 | 0.00 | 0.00 | 0.00 | −0.01 | −0.01 | −0.02 | |
| Wulbareg | Z | − 2.09* | −1.63 | −1.68 | 0.35 | 0.78 | 0.70 | −1.43 | −1.06 | −1.08 |
| | −0.03 | −0.03 | −0.03 | 0.01 | 0.01 | 0.01 | −0.02 | −0.01 | −0.01 | |
| Hawassa | Z | 0.15 | −0.78 | −0.85 | 0.10 | −0.55 | −0.63 | 0.20 | −0.58 | −0.75 |
| | 0.00 | −0.01 | −0.02 | 0.00 | −0.01 | −0.01 | 0.00 | −0.01 | −0.01 | |
*, **, indicate at 5 and 1%, significant levels, respectively.