IJERD – International Journal of Environmental and Rural Development (2022) 13-1

Research article
erd

Impact Analysis of Population Growth and Fertilizer Use on
Nitrogen Runoff in Nam Ngum Basin, Laos
KOSHI YOSHIDA*
Graduate School of Frontier Sciences, The University of Tokyo, Chiba, Japan
Email: kyoshida@edu.k.u-tokyo.ac.jp

ISSAKU AZECHI
Graduate School of Bioresources, Mie University, Mie, JapanTOSHIAKI IIDA
Faculty of Agriculture, Iwate University, Iwate, Japan
Received 29 January 2022

Accepted 2 May 2022(*Corresponding Author)

Abstract Human population growth has led to increased energy and food production,
fertilizer usage and wastewater flows. Increased nitrogen availability is a worldwide causeof eutrophication of rivers, lakes and estuaries, however, quantitative evaluation the impactsof nitrogen loads has been insufficient in developing countries because of poor data
availability. The Nam Ngum River basin, Laos, which supplies quality water for domestic
use in the Vientiane Metropolis, was selected as the target area for this study. The Nam
Ngum River basin is 415 km long with a 17,000 km2 catchment area, and the main land
uses in 2000 were paddy (15.6%), forest (32.3%), shrub (40.3%) and urban (0.2%). By UN
estimation, population in Laos is expected to increase 2.1 times from 2000 to 2050, andfertilizer use also will increase to produce sufficient food. Therefore, future water quality isa main concern in this river basin. Meteorological and hydrological data from 1995 to 2004,
and spatial data such as topography, land use, and soil properties were collected for model
simulation. A conceptual nitrogen balance model with three nitrogen pools was developed
and combined with a rainfall runoff model. Simulated river discharge and nitrogen loads
agreed with the observed data. Then, we investigated future nitrogen load variations in thebasin under different population growth and agricultural modernization scenarios. As aresult, even when population in the basin increased 2.1 times, nitrogen load did not change
significantly (11,676 tons/year in 2000 and 11,822 tons/year in 2050). However, thefertilizer increase scenario, from 25 kg/ha/season to 50 kg/ha/season, showed significant
increase in nitrogen loading, from 11,676 ton/year to 17,010 ton/year. Our results provide
initial insight into the magnitude and spatial distribution of nitrogen loading in Nam Ngum
River Basin, showing that this type of model may be useful for future impact assessments.
Keywords nitrogen load, point sources, diffuse pollution, agricultural modernization

INTRODUCTION
As the population increases, especially in developing countries, there is a need to increase foodproduction. This can be achieved in two ways: by expanding farmland and by increasing the yield
per unit area. The area of agricultural land in the world has already begun to decrease due to soil
degradation and other reasons, thus it is difficult to expect a significant increase in the area ofagricultural land. Therefore, the most realistic approach is to increase the yield per unit area (i.e.
increase the number of crops planted per year by switching to high-yielding varieties andintroducing irrigation systems). The introduction of high-yielding varieties, known as the Green
Revolution, has increased agricultural productivity, but chemical fertilizer inputs and advanced
management of agricultural water (irrigation and drainage) have become prerequisites forcultivation of such crops. As a result, pollution in closed water bodies in Asia has become aproblem due to excessive nutrient runoff caused by rapid population increase and the massiveⒸ ISERD

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application of chemical fertilizers. The water quality problem is particularly serious in the rapidly
developing tropical Asian monsoon region, where more than 30% of the world's chemicalfertilizers are applied. Because the population is still increasing, and it is necessary to develop
infrastructure to achieve stable food production and water environmental conservation (Tanaka etal., 2013). Normally, water quality is observed at the mainstream only, limiting understanding of
the spatial distribution of nutrient runoff from monitoring surveys alone. Additionally, it is difficult
to understand the spatial and temporal distribution of water resources and nutrient loading indeveloping countries due to low observation density and lack of observation data at the tributarylevel. A variety of methods have been used to model nitrogen transfer in river basins. Conceptualand physically based models (Lee et al., 2006; Conan et al., 2003; Whitehead et al., 2006) describe
the processes responsible for nitrogen wash-off into surface water and leaching to groundwater inlarge heterogeneous basins. Such models allow forecasting and a better understanding of processes.
However, even if these facts are well known, few scientific works on water quality in developing
countries have been published until recently, because available data are quite limited in suchregions. In this study, a water cycle and nitrogen dynamics model, which considered the local
cultivation and water treatment system, was developed and applied to a tropical Asian monsoon
basin with a paddy rice culture similar to that of Japan.
METHODOLOGY
Study Area
The Nam Ngum River basin in Laos is a tributary of the Mekong River. The river is about 415 kmlong with a catchment area of about 17,000 km2. The Nam Ngum 1 dam has an effective storage
capacity of 4.7 billion m3, located in the middle stream of the basin (Fig. 1).

Fig. 1 Nam Ngum River basin
The Nam Ngum 1 dam is dedicated to power generation, and supplies electricity to the Laos
capital, Vientiane, as well as exporting it to Thailand. The estimated basin population is 420
thousand persons and the average population density of the basin is about 25 people/km2. The basin
area accounts for 7% of the total land area and the basin population accounted for 9% of the total
population of Laos in 2000. The sewerage penetration rate in Laos is estimated to be 19.2%, but
this is mainly in urban areas.
Modernization in the agricultural sector is also lagging, with an irrigation rate of only 11%.
According to interviews in farming villages near Vientiane, the amount of fertilizer applied is stillⒸ ISERD

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low, averaging 25 kgN/ha/season, and some farmers do not apply any fertilizer in rainfed paddy
fields. The land use in the basin is paddy fields (16%), forests and bushes (72%), and residential
land (only 0.2%). The Nam Ngum 2 dam (effective storage capacity: 2.6 billion tons, completed in2011) and the Nam Ngum 5 dam (effective storage capacity: 250 million tons, completed in 2012)
are already in operation in the basin (Kudo et al., 2013), but this study considered only the Nam
Ngum 1 dam, because the meteorological, hydrological data required for the model simulation, andcollected from the hydro yearbook published by the Mekong River Commission, were only
available from 1995-2004. Water samples were collected monthly at Pakkanhoung station and thetotal nitrogen concentration was measured at the National University of Laos in 2003-2004.
Additionally, a field survey on water use, water treatment, irrigation, and cultivation methods wereconducted in the KM6 irrigation area in Nam Ngum basin. The factors affecting the water
environment can be broadly classified into point source loadings from people and livestock andnon-point source loadings from forest, farmland and urban areas. For the point source load,
statistical data, such as those on population and number of livestock, were collected from the
Bureau of Statistics, and the units of nitrogen loading in the Mekong River basin were used
(Tanaka et al., 2013). In Laos, households use septic tanks, is the simplest facility in sewage
treatment. The tanks receive domestic wastewater, which is treated by sedimentation and anaerobic
decomposition before the supernatant liquid is discharged into the drainage river. In Laos, thetemperature is high all year round, and the active decomposition of microorganisms is expected to
rapidly decompose excreta, but the treatment efficiency is extremely low due to lack of proper
maintenance. Livestock excrement is directly disposed in farmland and grassland, then discharged
into the ground water system after decomposition and absorption processes in the surface soil.
Rainfall-Runoff Model
To evaluate nitrogen transportation according to the river water flow, a distributed water cycling
model was developed and applied to analyze the water balance in the basin. TOPMODEL wasemployed for the rainfall-runoff analysis. Such a distributed model can include the spatial
distributions of topography, land use, and soil characteristics. Therefore, TOPMODEL is widelyused for hydrological characteristic analysis, water management, water quality analysis, and futureforecasting. TOPMODEL was proposed by Beven and Kirkby (1979) based on the contributing
area concept in hill slope hydrology. This model is based on the exponential transmissivity
assumption, which leads to a topographic index ln(a/To/tanb), where a is the upstream catchment
area draining across a unit length, To is the lateral transmissivity under saturated conditions, andtanb is the local gradient of the ground surface. Fig. 2 illustrates the conceptual structure of thewater cycle as estimated by TOPMODEL. Additionally, a dam operation model was combined with
TOPMODEL to calculate water storage in the reservoirs (Hanasaki et al., 2007). For details, please
see Yoshida et al., 2017.
Nitrogen Balance Model
A conceptual nitrogen balance model considering three pools in soil such as organic N, Ammonium
N and Nitrate N was developed for this study, as shown in Fig. 3. Soil N, mainly present in organic
form, is almost unavailable to plants.
The vegetation mainly uses inorganic forms of N, which are made available by organic matterdecomposition. Soil microorganisms convert the N contained in organic matter in a process calledmineralization. Although plants can use both forms of inorganic N, Nitrate is preferred because ofits greater solubility in water. In other words, nitrates quickly dissolve in the pore solution, which istaken up by plants. On the other hand, this also means that nitrate is easily leached to groundwater.
Ammonium N is less mobile because it strongly adsorbs on clay minerals due to its positive charge.
Denitrification is the anaerobic microbial reduction of N, and is used as an electron acceptor,
resulting in the transfer of soil nitrogen to the atmosphere.
The budget of each pool is expressed in the following equations.
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(1)

(2)

(3)
where Norg is organic nitrogen (kg/ha), PSNL is input N from point-sources (kg/ha/day), MinerN is
the mineralization rate (kg/ha/day), RunoffN is the nitrogen loss by surface runoff (kg/ha/day),Nammo is ammonium nitrogen (kg/ha), DepoN is nitrogen deposition (kg/ha/day), FertiN is
fertilized nitrogen (kg/ha/day), NitriN is the nitrification rate (kg/ha/day), Nnitra is nitrate nitrogen(kg/ha), PupN is the plant uptake rate (kg/ha/day), DenitN is nitrate nitrogen loss by denitrification(kg/ha/day), and LeachN is nitrate nitrogen loss by leaching (kg/ha/day). For details on thecalculation methods for each nitrogen flux and denitrification process, please see Yoshida et al.,2017.

Fig. 2 TOPMODEL structure

Fig. 3 Structure of the nitrogen balance model in soil
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Future Population Growth and Fertilizer Increase Scenarios
According to the United Nations prediction of 2000, the Laos population will increase 2.1 times by
2050. At the same time, chemical fertilizer use will also increase to produce sufficient food byagricultural modernization. This study evaluated the impacts of such population and fertilizer
increases on nitrogen loading in the Nam Ngum River basin. In 2000, the total population of Nam
Ngum basin was 420 thousand. Therefore, a population increase of 460 thousand people by 2050
was assumed and used for impact analysis. Additionally, we assumed that fertilizer use would
increase from 25 kgN/ha/season to 50 kg/ha/season to check the sensitivity of the model.
RESULTS AND DISCUSSION
Using the proposed model, water and nitrogen balances from 1995 to 2004 in the Nam Ngum Basin
were calculated at a 1 km × 1 km resolution. The first 5 years of data were used for parameter
calibration and the latter 5 years of data were used for validation. Parameters were calibrated bytrial-and-error method to maximize the Nash-Sutcliffe efficiency (NSE) of discharge and total
nitrogen (TN) concentration at Pakkanhoung (catchment area: 14,300 km2) station. Fig. 4 shows
the observed and calculated river discharge at Pakkanhoung station. The estimated NSE in thecalibration and validation periods were 0.54 and 0.50, respectively.

Fig. 4 Calculated and observed river discharge at Pakkanhoung station

Fig. 5 Calculated and observed TN concentration at Pakkanhoung stationⒸ ISERD180

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Model performance can be evaluated as “satisfactory”, if NSE > 0.50 (Moriasi et al., 2007).
The reason for the relatively low accuracy is that the input rainfall was only one point at
Pakkanhoung station. Fig. 5 shows a comparison of the calculated and measured TN concentrations
at Pakkanhoung station. The data used in this study were TN concentrations that were measured by
the authors from 2003-2004; the observed and calculated TN concentrations are compared here.
The correlation coefficient between the calculated and observed values was R = 0.75, indicatingthat the calculated values were slightly overestimated compared to the observed values. The
estimated average annual nitrogen load at Pakkanhoung station was 11,676 t/year and 8.1
kg/ha/year per unit area. Fig. 6 shows the spatial distribution of the annual mean TN concentration(1995-2004) and population density (persons/km2) map in 2000. In the Nam Ngum River basin, thetotal nitrogen concentration was almost less than 1 mg/L because of low population density andlow fertilizer use in the farmland. This analysis found that the total nitrogen concentration was notaffected by the spatial distribution of population density but was calculated to be relatively high inthe farmland during simulation periods.

Fig. 6 Spatial distribution of (a)annual mean total nitrogen concentration and(b) population density (persons/km2)

Fig. 7 Change in annual mean nitrogen load under the future scenarios
Fig. 7 shows the change in annual mean nitrogen load under the population growth andfertilizer increase scenarios. The population of the Nam Ngum River basin was 420 thousand in2000 and will increase 2.1 times to 860 thousand in 2050. The nitrogen load did not change
significantly (11,676 t/year in 2000 and 11,822 t/year in 2050) as a result of population increase in
the basin. The maximum population density in the Nam Ngum River basin in 2000 was about 2,140
persons/km2, with most people living in rural areas. Even if the population were to increase by 2.1
times, the population density would remain low; thus, there would be no significant impact on theⒸ ISERD

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water quality environment of the basin. The fertilizer increase scenario, however, from 25
kg/ha/season to 50 kg/ha/season, showed a significant increase in nitrogen loading, from 11,676 t/year
to 17,010 t/year. The current fertilizer application rate is 25 kgN/ha, which is extremely low. To
increase food production in response to future population growth, it is necessary to increase
production per unit area. Therefore, we assumed a scenario of increasing the amount of applied
fertilizer and evaluated the impact on the water quality environment of the watershed. Doubling thefertilizer application rate to 50 kg/ha resulted in a 46% increase in the annual nitrogen load at
Pakkanhoung station. The nitrogen discharge load per unit area increased from 8.1 kg/ha/year(equivalent of 0.66 mg/L) to 11.8 kg/ha/year (equivalent of 0.95mg/L) under this scenario, however,
the water quality environment was still favorable.
CONCLUSION
The Nam Ngum River basin, Laos, which supplies high quality domestic water to Vientiane
Metropolis, was selected as the target area for this study. The UN has estimated that the populationin Laos will increase by 2.1 times between 2000 and 2050, and producing sufficient food for thehigher population will require increased fertilizer use. Therefore, changes in future water quality is
the main concern for this river basin. Meteorological and hydrological data from 1995 to 2004, andspatial data such as topography, land use, and soil properties were collected for model simulation.
A conceptual nitrogen balance model with three nitrogen pools was developed and combined withthe rainfall runoff model. Simulated river discharge and nitrogen loading agreed with the observed
data. Next, we investigated future variations of nitrogen loading in the basin under the population
growth and agricultural modernization scenarios. As a result, even when population in the basin
increased by 2.1 times, the nitrogen load did not change significantly (11,676 t/year in 2000 and
11,822 t/year in 2050). However, the fertilizer increase scenario, from 25 kg/ha/season to 50
kg/ha/season, showed a significant increase in nitrogen load from 11,676 t/year to 17,010 t/year.
Our results provide a first insight into the magnitude and spatial distribution of nitrogen loading in
Nam Ngum River Basin. This type of model may be useful for future impact assessments.
ACKNOWLEDGEMENTS
This research was supported by JST SICORP Grant Number JPMJSC20E3, and the MEXT/JSPS
Grant-in-Aid for Scientific Research (KAKENHI) no. 19H03069 and no. 20KK0346.
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