# Application of the Standardised Streamflow Index for Hydrological Drought Monitoring in the Western Cape Province, South Africa: A Case Study in the Berg River Catchment

^{1}

^{2}

^{3}

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Study Area and Data

#### 2.1. Study Area

^{2}Berg River Catchment (BRC) (Figure 1), one of the two catchments in the Berg-Olifants Water Management Area (WMA). The BRC supplies water to parts of the WC province in SA. As shown in Figure 1, the Berg River in the BRC forms at the Franschhoek mountains and flows northwards, where it is joined by the Klein Berg, and then flows westwards until it discharges into the Atlantic Ocean. With a total length of approximately 285 km, the Berg River has up to nine major and seven minor tributaries. Six of these minor tributaries, which include the Klein Berg River, are perennial [1,5]. Surface water is a major water source in the BRC. The Mean Annual Runoff (MAR) in the upper Berg River and its tributaries is approximately 277 × 106 m

^{3}, approximately 263 × 106 m

^{3}at the upper middle Berg River and its tributaries, approximately 288 × 106 m

^{3}at the lower middle Berg River, 97 × 106 m

^{3}at the lower Berg River and its tributaries, and approximately 17 × 106 m

^{3}at the flood plain and estuary [1,5,45]. Thus, monitoring hydrological drought using streamflow is crucial in the BRC. The WC province experiences both winter, summer, and all-year rainfall. The annual rainfall in the WC ranges between 300 mm and 900 mm. The BRC is situated in the winter rainfall zone of the WC province, with a maximum rainfall of approximately 30 mm in June [1,5,45].

#### 2.2. Streamflow Data

^{3}. The G1H013 is in the upper middle Berg River, with a MAR of approximately 263 × 10

^{6}m

^{3}. The G1H008 is in the Klein Berg River on the lower middle Berg River, with a relatively low MAR of 263 × 10

^{6}m

^{3}. The Klein Berg River is a tributary of the Berg River (Figure 1 and Table 1). An assessment of the historical data obtained from G1H020, G1H013, and G1H008 indicates that those located on the middle and upper Berg River record relatively higher flows than those located on the Klein Berg River (Figure 2). Hence, in this study, to test the sensitivity of the SSI to various PDFs, streamflow time series were acquired from three streamflow gauging sites: G1H008, located on the low flow Klein Berg River; G1H013 located on the relatively high-flow lower part of the Berg River; and G1H020 also located on the relatively high-flow upper part of the Berg River.

#### 2.3. Rainfall Data

## 3. Methods

#### 3.1. SSI Calculation

_{0}= 2.515517; C

_{1}= 0.802853; C

_{2}= 0.010328; d

_{1}= 1.432788; d

_{2}= 0.189269; and d

_{3}= 0.001308 are constants. If the PDF, F(x), is suitable for fitting the monthly streamflow series, the average value of the SSI and the standard deviation must equal 0 and 1, respectively [40].

#### 3.2. PDFs Considered for SSI Calculation

Probability Distribution Function (PDF) Used for SSI Calculation in the BRC | PDF Equations | |
---|---|---|

Log-logistic [40,46] | $F\left(x\right)={\left[1+{\left(\frac{\alpha}{x-\gamma}\right)}^{\beta}\right]}^{-1}$ | (2) |

$\beta =\frac{2{w}_{1}-{w}_{0}}{6{w}_{1}-{w}_{0}-6{w}_{2}}$, $\alpha =\frac{\left(w-2{w}_{1}\right)\beta}{\Gamma \left(1+\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$\beta $}\right.\right)\Gamma \left(1-\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$\beta $}\right.\right)}$, $\gamma ={w}_{0}-\alpha \Gamma \left(1+\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$\beta $}\right.\right)\Gamma \left(1-\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$\beta $}\right.\right)$ | ||

Log-Normal [40,46] | $F\left(x\right)=\theta \left(\frac{ln\left(x-a\right)-\mu}{\sigma}\right)$ | (3) |

θ ≈ standard normal cumulative distribution function. $\sigma =0.999281z-0.006118{z}^{2}+0.000127{z}^{5}$ such that $z=\sqrt{\frac{8}{3}{\theta}^{-1}}\left(\frac{1+{\tau}_{3}}{2}\right)$. $\mu =ln\left[\frac{{\epsilon}_{2}}{\mathrm{erf}\left(\frac{\sigma}{2}\right)}\right]-\frac{{\sigma}^{2}}{2}erf$ is the Gauss error function such that $\mathrm{erf}\left(\frac{\sigma}{2}\right)=2\theta \left(\frac{\sigma}{2}\sqrt{2}\right)-1$ and $a={\epsilon}_{1}-{e}^{\mu +\frac{{\sigma}^{2}}{2}}$. | ||

Pearson Type III [40,46] | $F\left(x\right)=\frac{1}{\alpha \Gamma \left(\beta \right)}{{\displaystyle \int}}_{\gamma}^{x}{\left(\frac{x-\gamma}{\alpha}\right)}^{\beta -1}{e}^{-\left(\frac{x-\gamma}{\alpha}\right)}$ | (4) |

If ${\tau}_{3}\ge \raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$3$}\right.$, then ${\tau}_{m}=1-{\tau}_{3}$, leading to $\beta =\frac{\left(0.36067{\tau}_{m}-0.5967{\tau}_{m}^{2}+0.25361{\tau}_{m}^{3}\right)}{\left(1-2.78861{\tau}_{m}+2.56096{\tau}_{m}^{2}-0.77045{\tau}_{m}^{3}\right)}$ If ${\tau}_{3}<\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$3$}\right.$, then ${\tau}_{m}=3\pi {\tau}_{3}^{2}$; such that $\beta =\frac{\left(1+0.2906{\tau}_{m}\right)}{\left({\tau}_{m}+0.1882{\tau}_{m}^{2}+0.0442{\tau}_{m}^{3}\right)}$, $\alpha =\sqrt{\pi}{\epsilon}_{2}\frac{\Gamma \left(\beta \right)}{\Gamma \left(\beta +\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.\right)}$ and $\gamma ={\epsilon}_{1}-\alpha $ | ||

Weibull [40,46] | $F\left(x\right)=1-{e}^{-{\left(\frac{x-m}{a}\right)}^{b}}$ | (5) |

$b=\frac{1}{\left(7.859C+2.9554{C}^{2}\right)}$, $C=\frac{2}{3-{\tau}_{3}}-0.6309$, $a=\frac{{\epsilon}_{2}}{G\left(1+\frac{1}{b}\right)\left(1-{2}^{-\frac{1}{b}}\right)}$$m={\epsilon}_{1}-a\Gamma \left(1+\frac{1}{b}\right)$ | ||

Gamma [34,38] | $g\left(x,\alpha ,\beta \right)=\frac{1}{{\beta}^{\alpha}\Gamma \left(\alpha \right)}\left({x}^{\alpha -1}{e}^{-\frac{x}{\beta}}\right)$ | (6) |

α > 0 and β > 0 are the estimated shape and scale parameters, x > 0 is the streamflow (m^{3}/s), and Γ (α) is the Gamma PDF such that, $\Gamma \left(\alpha \right)={{\displaystyle \int}}_{0}^{\infty}{x}^{\alpha -1}{e}^{-x}dx$. |

#### 3.3. SSI Computation Using R Software Package

#### 3.4. Evaluation of Best Fitting PDFs for SSI Computation

#### 3.5. Evaluation of the Correlation between the SSI Computed Using the Selected PDFs

## 4. Results

#### 4.1. SSI Calculation Using the Selected PDFs

#### 4.2. The S-W Test for Normality on the SSI Calculated Using the Selected PDFs

#### 4.3. Visual Inspection of the SSI Calculated Using the Selected PDFs

#### 4.4. Evaluation of the Correlation between the SSI Computed Using the Selected PDFs

#### 4.5. Comparison of the SSI with SPI Results

#### 4.6. Drought Assessment Using the SSI Calculated Using the Gamma, Log-Normal and Weibull PDFs

## 5. Discussion

## 6. Conclusions

## Author Contributions

## Funding

## Informed Consent Statement

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

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**Figure 2.**Historical streamflow patterns (1990–2022) at gauging stations G1H008, G1H013, and G0H020 on the Berg and Klein Berg rivers in the Berg River Catchment (m

^{3}/s ≅ cumecs).

**Figure 3.**Historical rainfall (mm) (1980–2021) patterns in Franschoek, Stellenbosch, and Malmesbury towns, located in and around the Berg River Catchment.

**Figure 4.**SSI12 results for G1H008 streamflow time series computed using Gamma, Log-Logistic, Log-Normal, PTIII, and Weibull Probability Distribution Functions.

**Figure 5.**SSI12 results for G1H013 Streamflow time series computed using Gamma, Log-Logistic, Log-Normal, PTIII, and Weibull Probability Distribution Functions.

**Figure 6.**SSI12 results for G1H020 streamflow time series computed using Gamma, Log-Logistic, Log-Normal, PTIII, and Weibull Probability Distribution Functions.

**Figure 7.**Example of visual inspection of the SSI12 calculated using the Gamma, Log-Logistic, Log-Normal, PTIII, and Weibull for the G1H008 streamflow gauging station; from July 2015 to July 2018.

**Figure 8.**SSI12 time series for G1H008 computed using Gamma, PTIII (PearsonIII), Log-Normal, Log-Logistic, and Weibull PDFs in the Berg River Catchment.

**Figure 9.**SSI12 time series for G1H013 calculated using Gamma, PTIII (PearsonIII), Log-Normal, Log-Logistic, and Weibull PDFs in the Berg River Catchment.

**Figure 10.**SSI12 time series for G1H020 computed using Gamma, PTIII (PearsonIII), Log-Normal, Log-Logistic, and Weibull PDFs in the Berg River Catchment.

**Figure 11.**Comparison between SPI12 time series for Franschoek and SPI12 time series for Stellenbosch.

**Table 1.**Streamflow gauging stations that were used to obtain river discharge data for SSI calculations in the Berg River Catchment.

Streamflow Gauging Station Identity | River | Location Coordinates (Latitude: Longitude) | Period (Years) |
---|---|---|---|

G1H008 | Klein Berg | −33.313889:19.074722 | 1990 to 2022 (32 Years) |

G1H013 | Berg | −33.130833:18.862778 | 1990 to 2022 (32 Years) |

G1H020 | Berg | −33.707778:18.991111 | 1990 to 2022 (32 Years) |

SPI/SSI Values | Drought Classification |
---|---|

≥2.00 | Extremely Wet |

1.50 to 1.99 | Severely Wet |

1.00 to 1,49 | Moderately Wet |

0.00 to 0.99 | Mildly Wet |

0.00 to −0.99 | Mild Drought |

−1.00 to −1.49 | Moderate Drought |

−1.5 to −1.99 | Severe Drought |

≤−2.00 | Extreme Drought |

**Table 4.**SSI12 computed using Gamma, Log-Logistic, Log-Normal, PTIII, and Weibull PDFs for the G1H008 station between November 2004 and June 2005.

G1H008 | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|

Month-Year | SSI12 Gamma | Drought Classification | SSI12 Log-Logistic | Drought Classification | SSI12 Log-Normal | Drought Classification | SSI12 PTIII | Drought Classification | SSI12 Weibull | Drought Classification |

November 2004 | −1.6 | Severe | −1.4 | Moderate | −1.5 | Severe | −1.3 | Moderate | −1.5 | Severe |

December 2004 to April 2005 | −1.6 | Severe | −1.4 | Moderate | −1.6 | Severe | −1.4 | Moderate | −1.6 | Severe |

May 2005 | −1.6 | Severe | −1.4 | Moderate | −1.6 | Severe | −1.4 | Moderate | −1.5 | Severe |

June 2005 | −1.6 | Severe | −1.4 | Moderate | −1.5 | Severe | −1.3 | Moderate | −1.5 | Severe |

Average | −1.6 | Severe | −1.4 | Moderate | −1.6 | Severe | −1.4 | Moderate | −1.6 | Severe |

**Table 5.**SSI12 computed using Gamma, Log-Logistic, Log-Normal, PTIII, and Weibull PDFs for the G1H008 station between December 2015 and April 2016.

G1H008 | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|

Month-Year | SSI12 Gamma | Drought Classification | SSI12 Log-Logistic | Drought Classification | SSI1 Log-Normal | Drought Classification | SSI12 PTIII | Drought Classification | SSI12 Weibull | Drought Classification |

December 2015 to March 2016 | −2.2 | Extreme | −1.6 | Severe | −2.3 | Extreme | −1.6 | Severe | −2.0 | Extreme |

April 2016 | −2.1 | Extreme | −1.6 | Severe | −2.2 | Extreme | −1.6 | Severe | −1.9 | Severe |

Average | −2.2 | Extreme | −1.6 | Severe | −2.3 | Extreme | −1.6 | Severe | −2.0 | Extreme |

**Table 6.**SSI12 computed using Gamma, Log-Logistic, Log-Normal, PTIII, and Weibull PDFs for the G1H013 station between November 2003 and May 2005.

G1H013 | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|

Month-Year | SSI12 Gamma | Drought Classification | SSI12 Log-Logistic | Drought Classification | SSI12 Log-Normal | Drought Classification | SSI12 PTIII | Drought Classification | SSI12 Weibull | Drought Classification |

November 2003 | −1.2 | Moderate | −1.2 | Moderate | −1.3 | Moderate | −1.2 | Moderate | −1.2 | Moderate |

December 2003 to May 2004 | −1.3 | Moderate | −1.3 | Moderate | −1.3 | Moderate | −1.3 | Moderate | −1.3 | Moderate |

June 2004 | −1.1 | Moderate | −1.2 | Moderate | −1.3 | Moderate | −1.2 | Moderate | −1.1 | Moderate |

July 2004 | −1.1 | Moderate | −1.2 | Moderate | −1.2 | Moderate | −1.1 | Moderate | −1.1 | Moderate |

August 2004 | −1.0 | Moderate | −1.0 | Moderate | −1.0 | Moderate | −1.0 | Moderate | −1.0 | Moderate |

September 2004 | −1.1 | Moderate | −1.2 | Moderate | −1.1 | Moderate | −1.1 | Moderate | −1.1 | Moderate |

October 2004 | −1.1 | Moderate | −1.1 | Moderate | −1.1 | Moderate | −1.1 | Moderate | −1.1 | Moderate |

November 2004 | −1.0 | Moderate | −1.1 | Moderate | −1.0 | Moderate | −1.0 | Moderate | −1.0 | Moderate |

December 2004 to April 2005 | −1.2 | Moderate | −1.2 | Moderate | −1.1 | Moderate | −1.1 | Moderate | −1.1 | Moderate |

May 2005 | −1.2 | Moderate | −1.1 | Moderate | −1.0 | Moderate | −1.1 | Moderate | −1.1 | Moderate |

Average | −1.2 | Moderate | −1.2 | Moderate | −1.2 | Moderate | −1.1 | Moderate | −1.2 | Moderate |

**Table 7.**SSI12 computed using Gamma, Log-Logistic, Log-Normal, PTIII, and Weibull PDFs for the G1H013 station between August 2017 and May 2018.

G1H013 | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|

Month-Year | SSI12 Gamma | Drought Classification | SSI12 Log-Logistic | Drought Classification | SSI12 Log-Normal | Drought Classification | SSI12 PTIII | Drought Classification | SSI12 Weibull | Drought Classification |

August 2017 | −2.3 | Extreme | −1.9 | Severe | −2.9 | Extreme | −2.2 | Extreme | −2.1 | Extreme |

September 2017 to October 2017 | −2.5 | Extreme | −1.8 | Severe | −3.0 | Extreme | −1.9 | Severe | −2.2 | Extreme |

November 2017 | −2.4 | Extreme | −1.8 | Severe | −3.0 | Extreme | −2.0 | Extreme | −2.2 | Extreme |

December 2017 | −2.6 | Extreme | −1.9 | Severe | −3.1 | Extreme | −2.0 | Extreme | −2.3 | Extreme |

January 2018 | −2.7 | Extreme | −1.9 | Severe | −3.2 | Extreme | −2.0 | Extreme | −2.3 | Extreme |

February 2018 | −2.8 | Extreme | −1.9 | Severe | −3.3 | Extreme | −2.1 | Extreme | −2.4 | Extreme |

March 2018 to April 2018 | −2.9 | Extreme | −1.9 | Severe | −3.4 | Extreme | −2.1 | Extreme | −2.5 | Extreme |

May 2018 | −2.7 | Extreme | −1.9 | Severe | −3.3 | Extreme | −2.1 | Extreme | −2.4 | Extreme |

Average | −2.6 | Extreme | −1.9 | Severe | −3.2 | Extreme | −2.1 | Extreme | −2.3 | Extreme |

**Table 8.**SSI12 computed using Gamma, Log-Logistic, Log-Normal, PTIII, and Weibull PDFs for the G1H020 station between July 2003 and June 2004.

G1H013 | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|

Month-Year | SSI12 Gamma | Drought Classification | SSI12 Log-Logistic | Drought Classification | SSI12 Log-Normal | Drought Classification | SSI12 PTIII | Drought Classification | SSI12 Weibull | Drought Classification |

July 2003 | −1.1 | Moderate | −1.2 | Moderate | −1.2 | Moderate | −1.1 | Moderate | −1.0 | Moderate |

August 2003 | −1.2 | Moderate | −1.2 | Moderate | −1.3 | Moderate | −1.2 | Moderate | −1.2 | Moderate |

September 2003 | −1.1 | Moderate | −1.1 | Moderate | −1.1 | Moderate | −1.1 | Moderate | −1.1 | Moderate |

October 2003 | −1.2 | Moderate | −1.2 | Moderate | −1.2 | Moderate | −1.2 | Moderate | −1.2 | Moderate |

November 2003 | −1.2 | Moderate | −1.2 | Moderate | −1.2 | Moderate | −1.1 | Moderate | −1.1 | Moderate |

December 2003 to May 2004 | −1.2 | Moderate | −1.2 | Moderate | −1.2 | Moderate | −1.2 | Moderate | −1.1 | Moderate |

June 2004 | −1.0 | Moderate | −1.0 | Moderate | −1.0 | Moderate | −1.0 | Moderate | −0.9 | Mild |

Average | −1.2 | Moderate | −1.2 | Moderate | −1.2 | Moderate | −1.1 | Moderate | −1.1 | Moderate |

**Table 9.**SSI12 computed using Gamma, Log-Logistic, Log-Normal, PTIII, and Weibull PDFs for the G1H020 station between July 2017 and May 2018.

G1H013 | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|

Month-Year | SSI12 Gamma | Drought Classification | SSI12 Log-Logistic | Drought Classification | SSI12 Log-Normal | Drought Classification | SSI12 PTIII | Drought Classification | SSI12 Weibull | Drought Classification |

July 2017 | −1.9 | Severe | −1.8 | Severe | −2.3 | Extreme | −2.1 | Extreme | −1.7 | Severe |

August 2017 | −2.2 | Extreme | −1.8 | Severe | −2.6 | Extreme | −2.0 | Extreme | −1.9 | Severe |

September 2017 | −2.3 | Extreme | −1.9 | Severe | −2.7 | Extreme | −2.0 | Extreme | −2.0 | Extreme |

October 2017 | −2.5 | Extreme | −1.9 | Severe | −2.9 | Extreme | −2.1 | Extreme | −2.1 | Extreme |

November 2017 | −2.4 | Extreme | −2.0 | Severe | −2.8 | Extreme | −2.2 | Extreme | −2.0 | Extreme |

December 2017 to January 2018 | −2.4 | Extreme | −1.9 | Severe | −2.9 | Extreme | −2.1 | Extreme | −2.1 | Extreme |

February 2018 | −2.6 | Extreme | −2.0 | Extreme | −3.0 | Extreme | −2.2 | Extreme | −2.1 | Extreme |

March 2018 | −2.7 | Extreme | −2.0 | Extreme | −3.1 | Extreme | −2.2 | Extreme | −2.2 | Extreme |

April 2018 | −2.6 | Extreme | −2.0 | Extreme | −3.0 | Extreme | −2.2 | Extreme | −2.2 | Extreme |

May 2018 | −2.4 | Extreme | −1.9 | Severe | −2.9 | Extreme | −2.2 | Extreme | −2.1 | Extreme |

Average | −2.4 | Extreme | −1.9 | Severe | −2.8 | Extreme | −2.1 | Extreme | −2.0 | Extreme |

**Table 10.**Shapiro-Wilk Normality Test results for SSI−12 time series calculated using Gamma, Log-Logistic, Log-Normal, PTIII, and Weibull PDFs on streamflow gauging stations G1H008, G1H013 and G1H020.

Shapiro-Wilk Test for Normality | |||||
---|---|---|---|---|---|

Gamma | Log-Logistic | PTIII | Log-Normal | Weibull | |

G1H008 | W = 0.97464 p-value = 3.06× 10 ^{−6} | W = 0.96785 p-value = 1.83× 10 ^{−7} | W = 0.97958 p-value = 3.062× 10 ^{−5} | W = 0.94277 p-value = 5.416× 10 ^{−11} | W = 0.94277 p-value = 5.416× 10 ^{−11} |

G1H013 | W = 0.9717 p-value = 8.386× 10 ^{−7} | W = 0.95804 p-value = 5.186× 10 ^{−9} | W = 0.97103 p-value = 6.346× 10 ^{−7} | W = 0.94802 p-value = 2.293× 10 ^{−10} | W = 0.97389 p-value = 2.14× 10 ^{−6} |

G1H020 | W = 0.99019 p-value = 0.01188 | W = 0.98243 p-value = 0.0001341 | W = 0.99018 p-value = 0.01177 | W = 0.97238 p-value = 1.186× 10 ^{−6} | W = 0.98534 p-value = 0.0006527 |

**Table 11.**Correlation statistics for the SSI time series computed using the Gamma, Log-Normal, PTIII, Log-Logistic, and Weibull.

G1H008 | ||||||
---|---|---|---|---|---|---|

SSI Gamma | SSI log-Logistic | SSI log-Normal | SSI PTIII | SSI Weibull | Franschoek SPI12 (Gamma) | |

SSI Gamma | 1 | |||||

SSI log-Logistic | 0.98223 | 1 | ||||

SSI log-Normal | 0.984512 | 0.947839 | 1 | |||

SSI PTIII | 0.986172 | 0.998936 | 0.953691 | 1 | ||

SSI Weibull | 0.991315 | 0.98774 | 0.977377 | 0.990254 | 1 | |

Franschoek SPI12 (Gamma) | 0.741493 | 0.750306 | 0.715999 | 0.752611 | 0.754136 | 1 |

G1H013 | ||||||

SSI Gamma | SSI log-Logistic | SSI log-Normal | SSI PTIII | SSI Weibull | Franschoek SPI12 (Gamma) | |

SSI Gamma | 1 | |||||

SSI log-Logistic | 0.989213 | 1 | ||||

SSI log-Normal | 0.991976 | 0.965913 | 1 | |||

SSI PTIII | 0.994384 | 0.998255 | 0.976084 | 1 | ||

SSI Weibull | 0.995609 | 0.995802 | 0.977003 | 0.998044 | 1 | |

Franschoek SPI12 (Gamma) | 0.803445 | 0.81042 | 0.782366 | 0.811022 | 0.810208 | 1 |

G1H020 | ||||||

SSI Gamma | SSI log-Logistic | SSI log-Normal | SSI PTIII | SSI Weibull | Franschoek SPI12 (Gamma) | |

SSI Gamma | 1 | |||||

SSI log-Logistic | 0.994149 | 1 | ||||

SSI log-Normal | 0.994499 | 0.980957 | 1 | |||

SSI PTIII | 0.997659 | 0.997659 | 0.987417 | 1 | ||

SSI Weibull | 0.992818 | 0.993924 | 0.975533 | 0.996049 | 1 | |

Franschoek SPI12 (Gamma) | 0.817348 | 0.827468 | 0.804458 | 0.82031 | 0.816487 | 1 |

**Table 12.**Drought assessment during the period between 1990 and 2022 using the SSI12 calculated using the recommended Gamma, Log-Normal, and Weibull PDFs for the G1H008 streamflow gauging station.

Streamflow Gauging Station | Drought Period | Average SSI12 | Drought Classification | ||
---|---|---|---|---|---|

Gamma | Log-Normal | Weibull | |||

G1H008 | June 2000 to June 2001 | −0.7 | −0.7 | −0.8 | Mild Drought |

September 2004 to May 2005 | −1.6 | −1.6 | −1.5 | Severe Drought | |

September 2015 to April 2016 | −2.1 | −2.2 | −2.0 | Extreme Drought | |

August 2017 to May 2018 | −2.6 | −2.9 | −2.3 | Extreme Drought | |

September 2019 to July 2020 | −1.3 | −1.3 | −1.2 | Moderate Drought |

**Table 13.**Drought assessment during the period between 1990 and 2022 using the SSI12 calculated using the recommended Gamma, Log-Normal, and Weibull PDFs for the G1H013 streamflow gauging station.

Streamflow Gauging Station | Drought Period | Average SSI12 | Drought Classification | ||
---|---|---|---|---|---|

Gamma | Log-Normal | Weibull | |||

G1H013 | August 2000 to June 2001 | −0.6 | −0.6 | −0.7 | Mild Drought |

August 2003 to May 2005 | −1.2 | −1.2 | −1.2 | Moderate Drought | |

October 2011 to July 2012 | −1.0 | −1.0 | −1.0 | Moderate Drought | |

August 2015 to June 2016 | −1.4 | −1.4 | −1.3 | Moderate Drought | |

July 2017 to June 2018 | −2.5 | −3.0 | −2.2 | Extreme Drought | |

July 2018 to February 2019 | −1.1 | −1.1 | −1.1 | Moderate Drought |

**Table 14.**Drought assessment during the period between 1990 and 2022 using the SSI12 calculated using the recommended Gamma, Log-Normal, and Weibull PDFs for the G1H020 streamflow gauging station.

Streamflow Gauging Station | Drought Period | Average SSI12 | Drought Classification | ||
---|---|---|---|---|---|

Gamma | Log-Normal | Weibull | |||

G1H020 | August 2000 to June 2001 | −0.4 | −0.4 | −0.5 | Mild Drought |

July 2003 to June 2004 | −1.2 | −1.2 | −1.1 | Moderate Drought | |

August 2011 to July 2012 | −1.3 | −1.3 | −1.2 | Moderate Drought | |

August 2015 to January 2016 | −1.1 | −1.1 | −1.1 | Moderate Drought | |

July 2017 to June 2018 | −2.4 | −2.8 | −2.0 | Extreme Drought |

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## Share and Cite

**MDPI and ACS Style**

Mukhawana, M.B.; Kanyerere, T.; Kahler, D.; Masilela, N.S.
Application of the Standardised Streamflow Index for Hydrological Drought Monitoring in the Western Cape Province, South Africa: A Case Study in the Berg River Catchment. *Water* **2023**, *15*, 2530.
https://doi.org/10.3390/w15142530

**AMA Style**

Mukhawana MB, Kanyerere T, Kahler D, Masilela NS.
Application of the Standardised Streamflow Index for Hydrological Drought Monitoring in the Western Cape Province, South Africa: A Case Study in the Berg River Catchment. *Water*. 2023; 15(14):2530.
https://doi.org/10.3390/w15142530

**Chicago/Turabian Style**

Mukhawana, Mxolisi Blessing, Thokozani Kanyerere, David Kahler, and Ndumiso Siphosezwe Masilela.
2023. "Application of the Standardised Streamflow Index for Hydrological Drought Monitoring in the Western Cape Province, South Africa: A Case Study in the Berg River Catchment" *Water* 15, no. 14: 2530.
https://doi.org/10.3390/w15142530