Assessment and Modeling of Surface Water Quality Dynamics in Awash River basin, Ethiopia
No Thumbnail Available
Date
2019
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Addis Ababa University
Abstract
Awash River has the most important economic values in Ethiopia. However, it has been
recognized as being impaired by high amount of various pollutants owing to waste released
from different socio-economic activities in its basin since the basin encompasses the main
urban, industrial and agricultural centers of the nation. However, investigation of pollution
level of the basin is necessary for decision makers to safeguard Awash River and its end users,
which has not been addressed yet. This research was therefore aimed at evaluating the status,
assessing the spatial-temporal dynamics and modeling surface water quality dynamics in
relation to different land use scenarios in Awash River basin.
Status of water quality of Awash River was evaluated with respect to drinking and irrigation
water uses by choosing 17 sample sites along the River based on accessibility and land use
severity and sampling was done twice in each of the dry and wet seasons. Then both onsite and
offsite water quality analyses were undertaken following standard procedures. After
comparing different water quality indices in use todate, Canadian council of ministers of
environment water quality index was applied to compute the water quality indices. The
drinking and irrigation water quality indices of the upper basin were 34.79 and 46.39
respectively, which were in the poor and marginal categories of the Canadian water quality
ranking. Similarly, the respective indices for the middle/lower basin, which were 32.25 and
62.78, lie in the same ranges of the ranking. Although the difference in the used dataset of the
two cases and natural purification in the course of the River might contribute to the difference
in WQI, it is generally conceivable that the water quality of the River is below the fair rank.
To assess the spatial and temporal variation of water quality in the basin, means of the 9 years’
(2005-2013) water quality dataset of 19 parameters from 10 stations in the basin were used.
After validating, normalizing and checking the sampling adequacy and internal consistency of
the data, principal component analysis was computed and four principal components were
generated. Factor loadings, correlations between variables and the principal factors as well
as between sites and the principal factors were tabulated. Agglomerative hierarchical
clustering done on the dataset resulted in four clusters based on similarity of water quality
characteristics. The Mann-Kendall’s two tailed trend test detected temporal trends for total
hardness in February over all sites and for most parameters in the basin in the 9 years period.
Spatial analysis of the 14 sampling sites of the basin showed that as one moves from upper to
lower parts of the basin, electrical conductivity, total hardness and chloride levels decrease in the dry season. However, total hardness slightly increases and total dissolved solids, chloride,
and sulfate content decrease in the rainy season. Cl
are maximized
respectively at before Beseka and Beseka in both seasons and Beseka, before Beseka and
Sodere spring are found to be important sites responsible for the spatial variation.
-
and EC/TDS/SO4
-
To see the relation between land use/land cover (LULC) and water quality in the basin, LULC
dynamics was assessed by using cloud-free LS 5 and 7 TM imageries of 1994, 2000 and 2014.
The images were captured from EROS center of USGS GloVis viewer and classified by
supervised classification coupled with maximum likelihood algorithm in ERDAS Imagine. The
dominant LULC of the eight identified land use types were agriculture, barren land, and shrubland
in the 3 years’ period. Built-up and water bodies were found to have increased and
decreased respectively by about 147% and 63% as one goes from 1994 to 2014.
Moreover, in line with the changes in land use specifically of urbanization and agricultural
intensification from 2000 to 2014, around which water quality have been analyzed, the
parameters EC, TDS, Alkalinity, TH, SO4
2-
, and NH3 were found to increase
monotonically. Mean values of water quality indicators such as EC, nitrate, and some anions
have been compared in the agriculture-dominated, industry-dominated and urban-dominated
land uses. As a result, EC within the urban and industrial land uses was found to be maximized.
Nitrate, on the contrary, is observed to be higher in agriculture-dominated land uses and
higher concentration of anions (bicarbonates and chlorides) and hardness have been
generated from urbanized areas.
+
-
, Na
+
, Cl
, K
This study also evaluated performance of the Soil and Water Assessment Tool (SWAT) by
modeling nitrate and phosphate at the basin scale. First, the model was set up using digital
elevation model (DEM), climate, soil, and land use data. Thereafter, overall performance of
the model was assessed by linking its outputs to the Sequential Uncertainty FItting Version 2
(SUFI2) procedure of the SWAT Calibration and Uncertainty Program (SWAT-CUP). The
most sensitive parameters for the flow and nutrients were identified using t-stat and p-values
from global sensitivity analysis of the SWAT-CUP. The goodness-of-fit of the monthly
calibration measured by coefficient of determination (R
), Nash-Sutcliffe Efficiency (NSE), and
root mean square error-observations standard deviation ratio (RSR) were respectively 0.79,
0.64 and 0.60 for flow; 0.73, 0.71 and 0.54 for nitrate and 0.77, 0.76 and 0.49 for phosphate.
During validation, R
2
2
, NSE and RSR were respectively 0.81, 0.52 and 0.70 for flow; 0.68, 0.63 and 0.61 for nitrate and 0.82, 0.81 and 0.44 for phosphate. The results suggested that the model
is promising to predict nutrients in the basin.
From the modeling, concentrations of nutrients were found to be both seasonally and spatially
variable. Sub-basins 4, 8, 13, 21 and 39 were hotspots both in 1994 and 2014 with respect to
exporting higher amounts TN and TP. From the temporal investigation of nutrients’ monthly
averages in the period from 1997 to 2014 of sub-basin 3, the rainy months (March, July and
August) export higher amounts. Basin-wide comparison of the monthly averages of nitrate,
phosphate, TN and TP losses from the model simulations with the 2000 and 2014 LU’s indicate
that the respective values were generally greater in 2014 than in 2000. From the trends of TN
and TP for each of the 53 sub-basins in 1994, 2000 and 2014, slight reduction was observed
for the year 2000 as compared to that in 1994. However, since the increment from 2000 to
2014 was significant, the overall trend from 1994 to 2014 was found to be positive (increasing).
Results of the study have applications of filling the existing knowledge gap, facilitating
informed decision making, using as a customizable framework for similar studies in other river
basins of the nation.
Description
Keywords
Agglomerative hierarchical clustering, Awash River basin, CCME WQI, Drinking, Ethiopia, Irrigation, Land use/Land cover, Mann-Kendall’s trend, model, nutrients, principal component analysis, spatial and temporal, SUFI2 algorithm, SWAT-CUP, Water Quality