1

Phd RESEARCH PROPOSAL

MODELING THE HYDROLOGICAL IMPACT OF RICE

INTENSIFICATION IN INLAND VALLEY IN BENIN

By

Alexandre-Eudes DANVI

Supervisors

February 2012

Dr. Sander Zwart

Prof. Dr. Bernd Diekkrüger

Dr. Simone Giertz

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Table of contents

Introduction……………………………………………………………………………… 3

1. Background and justification………………………………………………………… 4

2. General methods…………………………………………………………………….. 7

3. Research questions…………………………………………………………………... 8

4. Research hypotheses…………………………………………………………………. 8

5. Study catchment area………………………………………………………………… 8

6. Site selection criteria………………………………………………………………… 10

7. Research methodology……………………………………………………………… 11

8. Materials…………………………………………………………………………… 17

9. Time table …………………………………………………………………………... 19

10. References…………………………………………………………………………....

.

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TOPIC: MODELING THE HYDROLOGICAL IMPACT OF RICE INTENSIFICATION IN

INLAND VALLEY IN BENIN

Introduction

Food production is gradually receiving more scientific and financial investments due to increasing

population growth with increasing global food demand and changing food preferences. From 2005 to

2050 an increase of 100–110% in global crop food demand has been forecasted (David et al, 2011),

and according to the World Bank, nearly 75 % of people facing dire poverty (earning less than 1USD

per day) in the developing countries rely on agriculture as their major source of food and income

(NRC, 2008). Based on the fact that production is still challenged by the world demand for food and

industrial crops (Charles et al, 2010), agricultural intensification is viewed as a key response for

helping to produce more food with the same amount of land.

Intensive agriculture is an agricultural production system that is characterized by an important use of

inputs for maximizing the production. Rice (Oryza sativa L.) is one of the intensively cultivated and

staple food crops in many African countries; and land under this crop is estimated to 161 million ha

worldwide (Monika et al, 2011). During the past three decades more importance is given to this

specific rice crop as for the development of strategic food security planning policies (Oteng et al,

1999). This is the case in West Africa where rice consumption is rising as well as production.

However, as the rate of the latter is not sufficiently satisfying the demand in the developing countries,

interest is taken more and more on options and technologies for intensification to improve the Paddy

yield and reduce imports. In the same logic, and for a purpose of sustainable and high rice production,

one rice ecotechnology system is being promoted and adopted in the inland valleys of Benin: the so-

called “Sawah” system of rice production through the project SMART-IV (Sawah, Market Access and

Rice Technologies for Inland Valleys).

With a relatively high demographic growth and an average per-capita income of 321 USD in 2006,

Benin is one of the world poorest countries whose economy is largely dependent on agriculture (Kuhn

et al, 2010). Benin has a huge potential in terms of water availability and agricultural lands ready to

answer a diversified and intensive agriculture. According to the number of FAO figures, only 4% of

the 322,000 ha irrigable lands are enhanced and in addition water management is not always

considered (Aquastat, 2005). Therefore, intensification of rice production is possible in Benin and

should record a high level of yield as demonstrated downstream areas of inland valleys in West Africa

which have shown considerable potential for sustainable intensification (Izac et al, 1993). Then, it

could be inferred that a best future is likely offered to the Sawah system implementation in Benin

wetlands.

The concept and the term “Sawah” refers to man-made improved rice fields with demarcated, leveled,

bounded and puddled rice fields with water inlets and outlets, which can be connected to various

irrigation canals, ponds, springs or pumps (Wakatsuki et al, 2008). For field operations, a total of

1,240mm is an average water requirement for an irrigated rice crop and factors to consider in water

management include reliable source of water, strong and clear water ways, well leveled and flat fields

and strong bunds (Buri et al, 2008). In addition, the system involves the use of fertilizers,

herbicide/pesticide application, and has been already tested in Ghana and Nigeria for its ability to give

a high yield.

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In spite of the positive outcome of increased yield, rice intensification systems also have associated

negative impacts. In upland areas, due to the irrigation channels network and the high potential for

unsustainable slash and burn, a risk of increased surface run-off, topsoil loss as well as erosion can be

run, and sub-surface water storage can be reduced. Downstream flooding can become more common

and downstream water supplies can be uneven, turbid and muddy (UNDP, 2010). Moreover, elevation

of nitrate concentration in stream and groundwater due to multi-year inputs from fertilizers and

manure via runoff might bring to water eutrophication. Using modeling tools, many previous case

studies have addressed the impact of rice cultivation on water environment in view of nitrogen (N)

flows in water systems (e.g Kee-An et al., 2006). Potential amount of N outflow from paddy fields has

been assessed based on water balance and input/output estimation of N in water flow at field scale.

Thus, the Sawah system development as rice intensification system may certainly impact the natural

resources. And the introduction of such technology in the inland valleys should be necessarily

followed with a thorough impact analysis which can be evaluated by a modeling approach.

In light of all above, this study entitled “Modeling the hydrological impact of rice intensification in

Inland valley in Benin” may be seen as one of the current research priorities expected for evaluating

newly developed agricultural technologies which are crucial to balance the increasing food demand.

This study is an alternative against threats to the present and future agriculture which requires an

integration of environmental health standards, economic profitability, and social and economic equity

(Charles et al, 2010). The general objective is to get a better understanding of the impact of the

agricultural management practices involved at field scale, when applying the Sawah system of rice

intensification, on water quantity and quality of inland valleys. This will contribute in improving

strategies for promoting an ecological and sustainable management of rice-growing ecosystems and

water resources of wetlands in Benin.

More specifically, it aims at:

i. Understanding the hydrological behavior of inland valleys depending on the major

hydrological processes occurring within the surrounding contributing watershed areas;

ii. Describing the hydrological characteristics of the rice intensification fields in the inland

valleys;

iii. Comparing APEX and SWAP models performance on simulating water and nitrate transport

on the rice fields;

iv. Assessing the water quality due to the nitrate concentration in surface water and the

intensification level within the inland valleys;

v. Simulating scenarios for analyzing long term impact on water quality based on the effect of

climate change and Land use change on nutrient loads.

1. Background and justification

Expansion and intensification of cultivation are among the predominant global changes of this century.

Intensification of agriculture by use of high-yielding crop varieties, fertilization, and irrigation has

contributed substantially to the tremendous increases in food production over the past 50 years

(Matson et al., 1997). Associated to irrigation, fertilizers have played an important role in increased

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crop production, especially cereal yields, and will continue to be a cornerstone of the science-based

agriculture required to feed the expanding world population (Balu et al, 1996). In Japan for instance,

paddy rice (Oryza sativa) culture is traditionally made to obtain either maximum yield or high quality

rice, or both by applying fertilizers even in excess of paddy nutrient requirements (Toshisuke et al,

2008).

As in Asia, rice has become the most important cereal crop in inland valleys of West Africa (Bado,

2010). West Africa is the most important region in Sub-Saharan Africa(SSA) in terms of rice

production (63%) and consumption (67%), followed by East Africa (production: 32%; consumption:

21%) and Central and South Africa (production: 5%; consumption: 12%), (WARDA, 2008). Though

total rice production in West Africa has increased by about 75 % over the last fifteen years through

many run projects, rice imports have increased a multiple thereof. In part, this situation is due to four

main problems identified previously by the FAO such as limited scope for the expansion of rain-fed

cultivation, rural labor shortages, high cost of modern irrigation, and incorrect readily proposed

transfer of the green revolution technology from Asia to Africa (FAO, 1986). The mentioned latter

problem can be more explained by the fact that new varieties of rice, one of the main crops that formed

for example the basis of the green revolution in Asia, yields well only if reliable rainfall or irrigation

provide sufficient moisture. As a constraint, large areas of West Africa suffer from highly unreliable

rainfall while irrigation is generally too costly for staple food production (Windmeijer and Andriesse,

1993).

In view of these considerations and in an attempt to improve rice production, inland valleys, which

occur so abundantly in West Africa’s countries like Benin, have appeared to have a high potential for

the development of rice-based smallholder farming systems at village scale. Study in Benin (e.g.

Speth et al. 2012; Giertz et al. 2008) revealed the importance of such valleys for food production. This

potentiality is mainly due to the specific hydrological conditions prevailing in the valley bottoms

where groundwater is at or near the surface during most of the year or seasonally, depending on the

climatological zone. And additionally in the transition between these valleys bottoms and the adjacent

uplands, lateral inflow of groundwater from the higher parts of the landscape effectively prolongs the

growing period for crops (Windmeijer and Andriesse, 1993). This explains somewhere the availability

of water during the dry season in some suitable wetlands of Benin, enabling then rice cultivation not

only during the rainy season but over the whole year.

In fact, extensive studies have been performed on inland valleys in West Africa. They have focused on

the agro-potential and the geomorphologic aspects of inland valleys, and great attention has been paid

to the classification and characterization of inland valley agro-ecosystems (Giertz et al, 2012).

Additionally to the characterization of inland valleys in Benin, most of the studies have been focusing

on planning and managing the exploitation of inland valleys by improving sustainable water resources

management strategies as well as agricultural practices, to be adopted at field scale in order to optimize

the cultivated crops production and face the rapid growth of the population. As examples, recent

studies have been dealing with determining constraints on the use of inland valleys ecosystems (Giertz

et al, 2012); investigating the inland valleys soil fertility potential for rice production (e.g Abe et al,

2010); assessing constraints and opportunities linked to the contribution of such valleys intensification

to sustainable rice cropping development (e.g Adetonah et al, 2010); assessing the soils water

dynamics of inland valleys and rice crop growth as affected by the use of water-saving and nutrient

management technologies such as bunding and fertilizer application for increasing rice production

(Nadine et al, 2012).

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However, it is important to notice that regarding water quality as far as Africa is concerned, studies on

nutrients export from rice fields are scarce and almost little have been done in Benin. More recently,

one study has been conducted by Bossa et al. (2012) who investigate the effects of crop patterns and

management scenarios on nitrogen (N) and phosphorus (P) loads to surface water and groundwater in

the Donga-Pont catchment a tributary of the Ouémé catchment. From this work, it has been found that

decreases in sediment and nutrient loads were induced by reductions in rainfall; and that the effects of

decline in rainfall were counteracted by the effects of land use changes. As conclusion, the results

indicate potential relationships between agriculture and water quality (Bossa et al., 2012a).

Indeed, there has been yet, much research carried out to evaluate the discharge of nutrient loads from

paddy rice culture and their impacts on downstream water bodies in Japan (e.g., Tabuchi and

Takamura, 1985; Tabuchi, 1986; Misawa, 1987; Kaneki, 1989; Ishikawa et al., 1992; Nagasaka et al.,

1998; Kyaw et al., 2005) and elsewhere (e.g., App et al., 1984 in Philippines; Maruyama and Tanji,

1997 in USA; Li and Yu, 1999 in China; Pathak et al., 2004 in Thailand). A general result found out

for instance, is that temporal changes in nutrient export and concentration in stream flow are subject to

some basic controls such as climate, nutrient availability, and agricultural activities (Pionke et al,

1999; Arheimer & Liden, 2000; Kemps & Dodds, 2001). Specifically, losses of Nitrogen occurring

from rice fields mainly through ammonia volatilization, denitrification, leaching, and runoff, not only

cause wastage of nitrogenous fertilizers, but have grave environmental consequences (Bandyopadhyay

et al, 2004) such as severe stream water eutrophication.

Eutrophication is mainly caused by excessive inputs of P and N and has many negative effects on

aquatic ecosystems. More explicitly, nitrate is transported in surface runoff, groundwater flow and

lateral soil flow, whereas organic N is assumed to be transported with sediment; and the major form of

P from paddy fields is sediment-bound (Somura et al, 2012). The most obvious impact on river or

stream water quality is the increased growth of algae and aquatic weeds that interfere with the use of

water for fishing, recreation, industry, agriculture and drinking (Carpenter et al, 1998). Moreover,

stream water quality is not the only one that is affected but also the groundwater. A high spatial

variation in the groundwater nitrate concentration, reaching 42mg/l has been observed for example

within the Donga-Pont catchment; and it has been clearly inferred that management practices such as

fertilizer inputs are among the principal factors controlling the dynamic of the strength relationship

between agriculture and water quality (Bossa et al. 2012a).

Beside the effect on water quality, changes in water quantity also occur from altered hydrological

processes at the watershed scale. Expansion of irrigated areas can result in groundwater overdraft and

streamflow depletion in some basins (Vorosmarty and Sahagian, 2000). Whereas irrigation reduces

some of the uncertainties related to the seasonal rainfall fluctuations and promotes increased

production, the land transformations involved (due to clearing, plowing, leveling, canals and bunds

construction for water distribution, etc...) and the quantity of water applied might disturb the local or

regional water balance.

Therefore, only thinking about intensification of rice cultivation to improve production, just because

resources are available and remain widely unexploited, is not sufficient. Beyond this and in case of

sustainable management, it should be also of interest to care about the modifications that are

undergoing with such practice and to determine the magnitude of the environmental impact. This will

help to access the relevant processes going around and to prevent as much as possible from an

environmental degradation. As an important decision made by AfricaRice Center in agreement with

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Bonn University for supervising this study through the SMART-IV project in Benin, it is a great step

towards filling this gap. At the end, it should be possible to describe how and define at which extent

rice intensification could impact the hydrological behavior of inland valleys by investigating the major

hydrological processes, their spatial and temporal variability that affect the generation of streamflow;

and analyzing the resulting changes of nitrogen concentration in water discharge from the watershed.

2. General methods

2.1. Concept of the Sawah ecotechnology system of rice intensification

Sawah ecotechnology is possibly the most promising rice production method because the sawah

system is already a highly productive and sustainable rice production system (Kyuma and Wakatsuki,

1995; Greenland, 1997). Authors have been concerned about the lack of appropriate terminology for

describing the rice growth environment using the term “Sawah fields” or “Paddy fields”. Lowland rice

fields are generally called ‘paddy fields’ in English but may also signify upland rice fields as rice is

grown in diverse biophysical environments along a toposequence (Andriesse and Fresco, 1991;

WARDA, 2004). These terminological uncertainties have been a considerable obstacle to the sharing

of ideas and strategies among researchers, policy makers and stakeholders about rice field

development. Abe and Wakatsuki (2011) therefore, propose the term ‘sawah’, which originates from

Malayo-Indonesian, to describe specifically man-made intensified rice fields with levelling, bunding

and puddling, in order to avoid any further terminological confusion when describing the rice growing

environment. The sawah has a levelled and puddled basin surrounded and thus demarcated by bunds.

It is often connected with irrigation and drainage facilities including a plot-to-plot irrigation/drainage

scheme and is submerged most of the time during the rice growth period. Note that the sawah does not

represent any special system such as the System for Rice Intensification (SRI), but a common system

often regarded as an Asian-type lowland rice field (Abe and Wakatsuki, 2011).

2.2. Watershed hydrological modeling

Modeling is a useful approach to understand present water quality conditions and to develop a decision

support system of watershed management (Somura et al., 2012). Predicting water availability, water

quality and sediment delivery at the watershed scale have become challenging issues for food supply,

food security, human health and natural ecosystems (Chaplota et al, 2005). In the past decades

significant progress has been achieved in understanding and modeling hydrological processes (Giertz

et al, 2003), and one of the more recent goals of hydrologic investigations in small catchments is to

understand better how streamflow is generated and how this process relates to water quality genesis.

In fact, progress in coupled hydrological/water quality modeling is more evident at the field scale or in

small homogeneous watersheds, while spatially-distributed modeling in larger watersheds represents a

more complicated problem. Conceptually, watershed simulation models describe mathematically water

fluxes and associated nutrient fluxes from land surface and soil profile to the closing river cross

section. It is widely known that watershed hydrology is dependent on many factors, including land use,

climate, and soil conditions. But the relative impacts of different types of land use on the surface water

are yet to be ascertained and quantified (Susanna and Wenli, 2002).

The consequences of agricultural land-use change for hydrologic processes include: changes in water

demands from changing land-use practices, such as irrigation; changes in water supply from altered

hydrological processes of infiltration, groundwater recharge and runoff; and changes in water quality

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from agricultural runoff (DeFries et al, 2004). However, identifying and quantifying the hydrological

consequences of land-use change are not trivial exercises, and are complicated by: (1) the relatively

short lengths of hydrological records; (2) the relatively high natural variability of most hydrological

systems; (3) the difficulties in ‘controlling’ land-use changes in real catchments within which changes

are occurring; (4) the relatively small number of controlled small-scale experimental studies that have

been performed; and (5) the challenges involved in extrapolating or generalizing results from such

studies to other systems (Beven, 2000). Moreover, the transfer of plot scale understanding to the

catchment scale requires the delineation and characterization of similar landscape elements

(hydrotopes or hydrologic response units) (Stefan, 2003) and field investigations concerning all

hydrological processes are essential (Giertz et al, 2003). The complexity of hydrological processes

depends on the environmental heterogeneity (e.g. soil distribution, topography, geology, vegetation,

anthropogenic impacts) and has to be analyzed in connection with the spatial scale (Bossa and

Diekkruger, 2012b).

3. Research questions

The main research question behind this study is: “Does the sawah system development for rice

intensification really allow a sustainable conservation of water resources in the Inland valleys?”

The specific research questions coming up are:

� How does the inland valley interact with its contributing watershed area?

� What are the spatial and temporal changes on water availability and quality in Inland valleys

due to the Sawah system development for rice intensification?

� How and through which major hydrological processes do these changes occur?

4. Research hypotheses

� Groundwater flow is the major hydrological process that contributes to surface water

availability and through which water quality is mainly affected.

� The influence of rice intensification on water quantity and quality depends on the location of

the rice fields and the level of intensification within the inland valley.

5. Study area

As mentioned before, this study will be carried out in Benin within the Ouémé catchment (Figure. 1)

and three inland valleys under different levels of rice intensification will be selected, and studied

individually with their associated contributing watersheds. The Ouémé basin is the largest river

catchment area in Benin and drains part of the northern, the central and the southern regions of the

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country (Anthony et al., 2002). It covers an area of approximately 50,000 km2 and drains to the south

into Lake Nokoue and flows through the coastal lagoon system into Gulf of Guinea (Speth et al, 2010).

Two main sub-basins can be observed: the “upper Ouémé basin “which is part of the dahomeyen

pediplain and the “Lower Ouémé basin” situated on the coastal sediments. Climatologically, the upper

Ouémé valley (north of 9oN) belongs to the Sub-humid Soudanian climate zone and littoral zone

(south of 7oN) belongs to the humid Guinea Coast. The Ouémé catchment records annual mean

temperature of 26 to 30oC and annual mean rainfall of 1,280 mm (from 1950 to 1969) and 1150 mm

(from 1970 to 2004) [Speth et al., 2010; Bossa et al., 2012b).

At regional scale, predominant soil are fersialitic soils (ferruginous tropical soils), characterized by

clay translocation and iron segregation (ferruginous tropical soils with concretions), which lead to a

clear horizon differentiation (Faure and Volkhoff, 1998; Gaiser et al., 2010; Bossa et al., 2012c). A

local scale description has shown a typical catena with soils formed on the slopes, leached ferruginous

tropical soils (Orthidystri-Epi- or Endoskeletic Acrisols/Haplic Lixisols or Typic Kandi-ustults/Typic

Kandiustalfs) (Busche et al., 2005; Junge, 2004; Sintondji, 2005, Hiepe, 2008; Gaiser et al., 2010;

Bossa et al., 2012c). Leached and indurated ferruginous tropical soils (Hyperalbi-Petric

Plinthosols/Plinthic Petraquepts) are developed at lower parts of the slopes. Hydromorphic soils

(Humic Gleysols/Typic Epiaquepts) are found in the inland valleys. In the riverbeds, poorly evolved

soils are distributed (Arenic Fluvisols/Typic Ustifluvents) (Junge, 2004; Sintondji, 2005, Hiepe, 2008;

Gaiser et al., 2010; Bossa et al., 2012c).

The landscape is characterized by forest islands, gallery forest, savannah, woodlands, and agricultural

as well as pasture land. Wetlands are found, and inland valleys that are regularly flooded during the

rainy season, are wide spread and have the potential to augment food production (Speth et al, 2010).

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6. Site selection criteria

The complexity of describing inland valleys hydrological behavior is well known by most of the

hydrologists; and then it is quite not simple to define clearly in the foreground all the required

characteristics that are suitable to lead this study. However, we outline below some major criteria for

selecting the representative sites. The general site selection criteria are valid for all sites while the

specific criteria are defined for each one of them. In doing so, we would like to emphasize that

selecting the sites will also depend on the realities that could be noted during fields’ survey and have

escaped to our attention.

General site selection criteria

Referring to each selected inland valley, the direct contributing watershed will be considered in this

study. Watersheds have an infinite variety of shapes, and the shape supposedly reflects the way that

runoff will “bunch up” at the outlet (Richard, 1998). Preferentially, small-scale watersheds are of

interest with size around or less than 10 km2 and agricultural land-uses within should be dominant.

The inland valleys should be closely localized along the major drainage axis of the different basins.

Well-marked toposequences should be observed with possibility to install instrument. The outlet

should be accessible at any time over the year for enabling hydrological measurements and its

longitudinal slope should allow for adequate drainage. The flood peak magnitudes of the valley bottom

Figure 1: Location of the study area and investigated gauged sub-catchments by IMPETUS

project (source: Bossa et al., 2012b).

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should not have a high effect by reducing major part of the area available for intensification and

restricting the farmers to not growing rice for a long period in the year (at least during the critical

periods of rice plants life such as transplanting and flowering). This can be roughly investigated by

discussing with the farmers in place.

Specific site selection criteria

- Site 1:

As one important dimension of agricultural intensification is the length of the fallow period, the first

site will be chosen with a negligible period of fallow between two rice cropping seasons. In this

considered most intensive system, the new rice growing season is requested to be undertaken shortly

after the harvest of the preceding one. In most of the case, the process is commonly coupled with high

rate of chemical fertilizer application, and crop water demand is assisted during the dry season either

with irrigation water traditionally derived from the stream by a canal, or a shallow groundwater

contribution (depending on the fields location along the slope and in order to stand the growing

period).

A valley stream with a regular flow and slight inundation magnitude is recommended of preference.

- Site 2:

The second site is taken as medium or intermediate level of intensification and shall feature at least an

annual rice cropping system. In this case, the rice is intended to be traditionally grown on the area

mostly during the rainy season either with the same or less amount of fertilizer application rate than

the previous site. This situation can be noted with farmers, either when the economic conditions don’t

allow for hosting ongoing rice cropping, or when there are some particular constraints related to the

climate (limited or exceeded water availability), the physical characteristics of the valleys, or to the

market access.

- Site 3:

For the third site, the major specific criterion is that no rice cultivation should be practiced. The area

can be grown preferentially with other crops in order to give it a neutral view regarding rice

agricultural intensification.

7. Research methodology

7.1. Literature review

A literature review of all developed concepts on water quantity and quality associated to wetlands and

taking into account hydrological processes and nutrients dynamics in an agricultural catchment will

be made. This will consist in conducting literature search on existing available source of document

related to the study area; published papers dealing with watersheds and riparian wetlands hydrology,

nutrient dynamics assessment, environmental risks of scarcity and pollution on water resources; and

any other document likely to be necessary for a best apprehension of the study issues.

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7.2. Sites instrumentation

For surveying the soils properties distribution, measuring the necessary meteorological and

hydrological parameters, creating data base and preparing the input files in order to run the model, the

experimental sites will be adequately equipped. Depending on the topography, geomorphology, soil

types and land use distribution in situ, the location to install each instrument will be defined following

to several field visits.

7.3. Hydrological Models

Providing knowledge and data for deep understanding of the interrelationship among meteorology,

surface water, groundwater, and the predominant physical factors that influence the flow and timing of

water in the inland valleys is one of the main focuses of this study. For achieving this goal, two

candidate hydrological models have been put under interest: APEX (Agricultural Policy

Environmental Extender) and SWAP (Soil-Water-Atmosphere-Plant system).

� APEX model description and approach

APEX is a continuous, daily time-step, hydrologic/water quality model that has been developed to

evaluate various land management strategies at scales ranging from field to farm or to small

watersheds (Figure. 2). The model helps for providing management tools in order to obtain sustainable

production efficiency and maintain environmental quality. The farm or small watershed scales may

involve several fields, or subareas, which are homogenous with respect to climate, soil, land use, and

topography (Williams et al., 2008). In addition to the Environmental Policy Integrated Climate (EPIC)

model functions, it has components for routing water, sediment (deposition and degradation), and

nutrients across complex landscapes and channel systems to the watershed outlet. Groundwater and

reservoir components are also included (Williams et al, 2006). The model simulates the groundwater

volumes based on a mass balance equation (Equation. 1) and the watershed outflow is computed by

combining the watershed storage and the simulated rainfall excess (Equation. 2).

GWST=GWST0+QV+SEP-DPRK-RSSF (1)

Where: GWST0 and GWST are the groundwater volumes at the start and end of the day in mm; QV is

the root zone percolation rate in mm d-1

; SEP is the reservoir seepage rate in mm d-1

; DPRK is the

percolation rate from the groundwater storage in mm d-1; and RSSF is the return flow rate in mm d-1.

qhy=(STH0*(1.0-exp(p73*DTHY/TC))+DQ)*WSA/(DTHY*360) (2)

Where: qhy is the watershed outflow rate in m3.s-1; STHO is the watershed storage volumes in mm at

the beginning of the time interval in h; p73 is a parameter (0.1<p73<1.0); DTHY is the time interval in

h; and TC is the watershed time of concentration in h; and WSA is the watershed area in ha.

Soluble N in the groundwater is considered conservative and daily content is calculated with the mass

balance equation:

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GWSN=GWSN0+RZLN-RSFN-GWLN (3)

Where: GWSN0 and GWSN are the initial and final soluble N contents in kg ha-1; RZLN is the root

zone N leaching rate in kg ha-1

.d-1

: RSFN is the return flow N rate in kg ha-1

.d-1

; and GWLN is the

groundwater N leaching rate in kg.ha-1

.d-1

.

The amount of NO3-N lost when water flows through a layer is estimated by considering the change in

concentration. Thus, the equation:

QNO3=QT*CNO3 (4)

Where; QNO3 is the amount of NO3-N lost from a soil layer and CN03 is the average concentration of

NO3-N in the layer during the percolation of volume QT through the layer.

In fact, GIS and other windows based interfaces such as ArcAPEX, i-APPEX and WinAPEX have

been developed to automate the input parameterization of the APEX model, and to facilitate its

applications for watershed and regional-scale assessments. The ArcAPEX interface is able to generate

a set of initial input parameters based upon the subarea delineation, subarea land use/soils/slope

analysis, and the weather data. These parameters are stored in tables within the ArcAPEX Project

geodatabase (Tuppad et al, 2009) and can be used in WinAPEX for further model simulation purpose.

I-APEX provides tool for managing large numbers of APEX runs, handling data input and output.

Figure 2: Overview of Apex Model structure.

a) Schematic of major agroecosystem processes simulated by the

Apex (Source: Powers et al., 2011).

APEXRain,

Snow,

Chemicals

Subsurface

Flow

Surface

Flow

Below Root

Zone

Evaporation and Transpiration

b) Land use types and routing process in Apex

(Source: Williams et al., 2006).

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� SWAP model description and approach

Developed by Alterra and Wageningen University, SWAP offers a wide range of possibilities to

address both research and practical questions in the field of agricultural water management and

environmental protection. The model is a comprehensive one-dimensional physically based model

designed to simulate flow and transport processes at field scale level, during growing seasons and for

long term time series. SWAP simulates transport of water, solutes, and heat in the saturated and

unsaturated top-soil layers, and in interaction with vegetation development (Figure. 3).

The model employs the Richards equation (Equation. 5) including root water extraction to simulate

soil moisture movement in variably saturated soils. Concepts are added to account for macroporous

flow and water repellency. SWAP considers for solute transport the basic processes convection,

dispersion, adsorption and decomposition.

(5)

Where; θ is volumetric water content (cm3.cm-3); t is time (d), K(h) is hydraulic conductivity (cm.d-1);

h is soil water pressure head (cm); z is the vertical coordinate (cm) taken positively upward; Sa(h)is

soil water extraction rate by plant roots (cm3.cm

-3.d

-1); Sd(h) is extraction rate by drain discharge in the

saturated zone (d-1); Sm(h) is exchange rate with macro pores (d-1).

In the vertical direction the model domain reaches from a plane just above the canopy to a plane in the

shallow groundwater. In this zone the transport processes are predominantly vertical; therefore SWAP

is a one-dimensional, vertical directed model. The flow below the groundwater level may include

lateral drainage fluxes, provided that these fluxes can be prescribed with analytical drainage formulas.

In the horizontal direction, SWAP’s main focus is the field scale. At this scale most transport

processes can be described in a deterministic way, as a field generally can be represented by one

microclimate, one vegetation type, one soil type, and one drainage condition(Van Dam et al, 2008).

15

7.4. Data required

1. Information on field operations and operation schedules including crops and crop rotations,

tillage, irrigation/drainage, amount and timing of fertilizer application; on livestock and on

grazing.

2. Information on the crops development (crop name, harvest index, leaf area index, maximum

root zone depth...), Nitrogen uptake, Nitrogen content in yield.

3. Meteorological data such as average values of the following measurable weather parameters:

maximum and minimum air temperature, precipitation, solar radiation, relative humidity, wind

velocity.

4. Spatial data sets, including a Digital Elevation Model and GIS maps of Land cover/land use

and soil.

5. Soil physical and chemical characteristics (Soil type, layers depth, bulk density, texture,

wilting and field capacity points, infiltration rate, hydraulic conductivity…); initial soil

moisture condition.

(Data for models calibration and validation)

6. Groundwater level.

7. Flow discharge of surface water at the outlet.

8. Nitrate concentration in surface water flow at the outlet.

Figure 3: Overview of the hydrological processes in the SWAP model (source: Van Dam, 2000).

16

Table 1: Data source and methodology of collection

Input data Data source Method of collection

1. Farmers, Official Agriculture

departments, DG-Eau, MAEP,

CERPA, CECPA, in situ

measurements.

Discussion with farmers during field surveys,

acquisition of available databases on the selected

sites, conducting field measurements.

2. Literature review, available

databases, in situ measurements.

Making literature review and conducting field

measurements.

3. ASECNA, AfricaRice, IRD,

IMPETUS project, and any

other Institute, Centers or

project; in situ measurements.

Available databases on climate parameters records

and field measurements by weather station

installation.

4. SRTM, IMPETUS project, IGN. Acquisition

5. Intensive measurements at different points and different depth along transects within

the watershed based on the soil types, geographical features and land use distribution;

and soil samples analyzed at laboratory.

6. In situ measurements Generating data by sites instrumentation and

continuous field measurements.

7. DG-Eau, MAEP, CERPA,

CECPA, IMPETUS Project.

Accessing available databases and doing

measurements at gauging stations.

8. Water sampling and determination of nitrate concentration in water samples at

laboratory.

17

7.5. General methodological approach for the study

8. Materials

Mainly the following materials and accessories listed below are proposed for field instrumentation:

� Three Climate stations for measuring climate variables and each with 2 additional rain gauges

as backup.

� Three Water level gauges for measuring water level at the outlet of the inland valleys.

� Six Groundwater level gauges for determining groundwater – surface water dynamic to study

water fluxes and water availability in the wetland;

� One Flow meter for establishing the rating curve and calculating discharge from the water

level measurements at the outlet of the inland valley.

� One Multiparameter probes for determining the nitrogen transport at the outlet of the inland

valley.

Figure 2: Methodological approach proposed for this study

18

(Accessories)

� Plastic sample bottles (high-density polyethylene, polypropylene, polycarbonate, or a

fluoropolymer) of 200mL to sample surface water.

� Refrigerator or freezer is required for storing the water sample bottles depending on sample

holding time and storage.

� Soil sampling auger and labeled containers for collecting soil samples.

� Tape measure; spade or shovel; mattock; stainless steel, plastic or other composition bucket.

� V-Notch weirs for quantifying water flow during irrigation and drainage processes on the

Paddy fields by measuring water head level on the weir.

� Double-ring infiltrometer for measuring infiltration capacity.

� One GPS.

19

9. Time table

Year 2012 2013 2014 2015

Month 9 10 11 12 1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 10 11

ACTIVITIES

Literature

review

Research

proposal

Site selection

&

instrumentation

Data collection

and fields

monitoring

Model

calibration/

validation

Writing

dissertation

Submission

dissertation/

Defense

In Germany

In Benin

In Germany In Germany

In Germany

Table 2: Program outline

18 months in Benin/22 months in Germany

In Benin

In Germany

20

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