Mapping and estimating water quality parameters in the Volta Lake’s Kpong Headpond of Ghana using regression model and Landsat 8

Figures & data

Figure 1. Study area showing the portion of the lake.

Table 1. Summary statistics of the physical and chemical parameters of the study area.

Parameter	Min	Max	Average	SD	WHO
pH	5.90	7.80	6.90	0.38	6.5–8.5
Alkalinity	24.40	34.70	27.90	8.60	500.00
Temperature	26.80	28.00	27.50	0.30	–
TSS	18.00	48.00	28.70	5.08	05.00
EC	0.026	0.17	0.06	0.02	1200.00
Turbidity	5.56	24.90	7.90	3.00	5
PO4-P	0.03	0.07	0.04	0.01	–
NO3-N	0.40	1.00	0.60	0.16	50
NH4-N	0.010	0.08	0.05	0.02	35
Chl-a	0.21	13.50	5.10	4.30	–
DO	2.02	6.60	5.60	0.67	8–10
Cl^−	0.00	12.00	2.18	1.21	200–300

All the measurements are in mg/L with the exception of pH (pH units); EC (µS/cm); turbidity (NTU).

Figure 2. (a) Remote sensing reflectance of Landsat band and in-situ measured Chl-a concentration for 17 December 2022. (b) Remote sensing reflectance of Landsat band and in-situ measured Chl-a concentration for 15 March 2023.

Table 2. Equations for regression models for Chl-a, turbidity, and TSS (using the blue, green and red bands, i.e. B2, B3 and B4, respectively).

Date	Parameters	Regression equation	Band combination	R	R^2	Bias	RMSE
17 December 2022	Chl-a	8176.2x + 1255.9	B3/B2	0.701	0.883	0.540	2.340
	Turbidity	62.98x + 4.67	B3	0.601	0.668	2.341	3.142
	TSS	8434.3x + 9379.2	B4/B1	0.683	0.753	3.213	3.542
15 March 2023	Chl-a	774.3In(x) + 1886.8	B2 + B3	0.772	0.853	0.256	1.573
	Turbidity	105.21x − 7.230	B3 + B4	0.598	0.701	2.975	3.682
	TSS	5324In(x) + 1433.2	B3/B4	0.618	0.712	1.971	3.276

Figure 3. (a) Remote sensing reflectance of Landsat band and in-situ measured TSS concentration for 17 December 2022. (b) Remote sensing reflectance of Landsat band and in-situ measured TSS concentration for 15 March 2023.

Figure 4. (a) Remote sensing reflectance of Landsat band and in-situ measured turbidity concentration for 17 December 2022. (b) Remote sensing reflectance of Landsat band and in-situ measured turbidity concentration for 15 March 2023.

Table 3. Statistics of validation results for in-situ and estimated water quality parameters.

Date	Parameters	Data source	Number of samples (n)	Min	Max	Mean	SD	CV (%)	SE
17 December 2022	Chl-a	In-situ measurement	20	0.226	12.365	6.252	3.826	40.23	1.73
		Landsat8 OLI	20	1.40	12.833	6.721	3.375	37.23	1.65
	Turbidity	In-situ measurement	20	6.800	24.4	17.233	1.767	36.34	0.59
		Landsat8 OLI	20	5.71	23.623	19.386	1.865	35.89	0.65
	TSS	In-situ measurement	20	20.01	48.00	42.565	8.092	11.35	7.45
		Landsat8 OLI	20	23.0	52.912	41.218	9.623	13.45	6.38
15 March 2023	Chl-a	In-situ measurement	20	0.206	13.235	5.987	2.626	36.63	0.54
		Landsat8 OLI	20	0.854	12.785	6.045	3.871	39.56	1.32
	Turbidity	In-situ measurement	20	5.567	22.786	18.123	1.986	36.43	0.67
		Landsat8 OLI	20	4.71	24.371	20.588	2.017	34.64	0.725
	TSS	In-situ measurement	20	20.876	48.651	41.865	10.752	13.542	8.653
		Landsat8 OLI	20	18.072	50.652	43.199	9.326	11.965	7.23

Figure 5. (a) Trends of observed and sensor-estimated Chl-a concentration for the first period. (b) Trends of observed and sensor-estimated Chl-a concentration for the second period.

Figure 6. (a) Trends of observed and sensor-estimated TSS concentration for the first period. (b) Trends of observed and sensor-estimated TSS concentration for the second period.

Figure 7. (a) Trends of observed and sensor-estimated turbidity concentration for the first period. (b) Trends of observed and sensor-estimated turbidity concentration for the second period.

Figure 8. (a) In-situ and estimated Chl-a distribution on 17 December 2022. (b) In-situ and estimated Chl-a distribution on 15 March 2023.

Figure 9. (a) In-situ and estimated TSS distribution on 17 December 2022. (b) In-situ and estimated TSS distribution on 15 March 2023.

Figure 10. (a) In-situ and estimated turbidity distribution on 17 December 2022. (b) In-situ and estimated turbidity distribution on 15 March 2023.

Figure 11. (a) Spatial extent of Chl-a, TSS and turbidity at five different land use sites. (b) Spatial extent of Chl-a, TSS, and turbidity at five different land use sites.

Data availability statement

The datasets generated during/or analyzed during the current study are available from the corresponding author upon reasonable request.