doi:10.5194/piahs-364-380-2014

380

Evolving Water Resources Systems: Understanding, Predicting and Managing Water–Society Interactions
Proceedings of ICWRS2014, Bologna, Italy, June 2014 (IAHS Publ. 364, 2014).

Evaluation of surface water dynamics for water-food security inseasonal wetlands, north-central Namibia
TETSUYA HIYAMA1, TETSUJI SUZUKI2, MIHO HANAMURA2,
HIROKI MIZUOCHI3, JACK R. KAMBATUKU4, JOHANNA N. NIIPELE4,
YUICHIRO FUJIOKA5, TAKESHI OHTA2 & MORIO IIJIMA5
1 Hydrospheric Atmospheric Research Center, Nagoya University, Nagoya, Japanhiyama@hyarc.nagoya-u.ac.jp
2 Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya, Japan
3 Graduate School of Life and Environmental Sciences, University of Tsukuba, Tsukuba, Japan
4 Faculty of Agriculture and Natural Resources, University of Namibia, Oshakati, Namibia
5 Faculty of Agriculture, Kinki University, Nara, Japan (co-affiliation: JST/JICA, SATREPS, Japan)

Abstract Agricultural use of wetlands is important for food security in various regions. However, land-use
changes in wetland areas could alter the water cycle and the ecosystem. To conserve the water environmentsof wetlands, care is needed when introducing new cropping systems. This study is the first attempt to evaluate
the water dynamics in the case of the introduction of rice-millet mixed-cropping systems to the Cuvelai systemseasonal wetlands (CSSWs) in north-central Namibia. We first investigated seasonal changes in surface water
coverage by using satellite remote sensing data. We also assessed the effect of the introduction of rice-millet
mixed-cropping systems on evapotranspiration in the CSSWs region. For the former investigation, we used
MODIS and AMSR-E satellite remote sensing data. These data showed that at the beginning of the wet season,
surface water appears from the southern (lower) part and then expands to the northern (higher) part of theCSSWs. For the latter investigation, we used data obtained by the classical Bowen ratio-energy balance(BREB) method at an experimental field site established in September 2012 on the Ogongo campus,
University of Namibia. This analysis showed the importance of water and vegetation conditions whenintroducing mixed-cropping to the region.
Key words Cuvelai system seasonal wetlands (CSSWs); flood- and drought-adaptive cropping system (FDACS);
Bowen ratio-energy balance (BREB) method; water environments

INTRODUCTION
Wetlands cover 6% of the Earth’s land surface and have great hydrological and ecologicalimportance. Agricultural use of wetlands is important for food security in various regions. However,land-use changes in wetland areas could alter evapotranspiration, a major component of the waterbalance, which could eventually affect the water cycle and ecosystems (Suzuki et al. 2013). To
conserve the water environments of wetlands, care is needed when introducing new cropping
systems. From the socio-ecological point of view, harmonisation between development andenvironmental conservation is a universal issue in the 21st century. Particularly in semi-arid regionsof Africa, where food scarcity is a serious problem, there is the risk of rapid and disorderly
agricultural development without consideration of the environment. In recent years, semi-arid
regions of sub-Saharan Africa have been struck periodically by severe droughts as well as severe
floods, which have caused food shortages in the region.
Namibia, located in southern Africa, has an area of 824 000 km2, a population of about
2 364 000, and a gross national income (GNI) per capita of 4510 USD (The World Bank 2012).
With its rich mineral resources, Namibia’s economic growth was 4.8% per year on average from2009 to 2010 (The World Bank 2012). However, Namibia is one of the least equitable countries inthe world, as shown by a Gini index from 64 to 74 (United Nations Development Programme 2007,
The World Bank 2012).
A quarter of the population lives in north-central Namibia, where most residents are subsistence
farmers cropping pearl millet and raising livestock. Mean annual precipitation in the region is about
400 mm. The wet season starts in the middle of October and ends in the middle of April of the nextyear. The dry season extends from mid-April to mid-October. In the wet season, surface (flood)water flows from the Angolan Plateau and creates a vast seasonal wetland covering approximately
800 000 ha. The region is locally called the Cuvelai system seasonal wetlands (CSSWs) (NjungeCopyright  2014 IAHS Press

Evaluation of surface water dynamics for water-food security in seasonal wetlands, Namibia

3812013). The amount of flood water has varied widely in the last 10 years, causing serious deluge ordrought in the region.
The water resources of the CSSWs are currently used mainly for grazing and fishing, and notfor cropping. The limited use of these water resources is due to the unstable flood intensity andbecause the Etosha Pan in the southern part of the CSSWs is a national wildlife sanctuary. If largescale agricultural development were to occur in this region, the vulnerable wetland environment
might be harmed (Kambatuku et al. 2013).
Since 2011, we have been conducting research aimed at developing a flood- and droughtadaptive cropping system (FDACS) that can preserve water environments and cope with yearlyfluctuations in rainfall. The FDACS will be developed through trials in the fields of crop science,development studies, hydrology, and integrated studies. This paper mainly focuses on the hydrology
aspects. The purposes of this study are: (1) to understand surface flooding around the CSSWs, and(2) to assess the effects of the introduction of rice-millet mixed-cropping systems on evapotranspiration in the region.
METHODS
To achieve research purpose (1), we used satellite remote sensing data of the Moderate Resolution
Imaging Spectroradiometer (MODIS) and the Advanced Microwave Scanning Radiometer-Earth
Observing System (AMSR-E). Using data from these two sensors, we investigated seasonal changes
in surface water coverage over the CSSWs. For research purpose (2), we newly established twoBowen ratio-energy balance (BREB) measuring systems in an experimental research field at Ogongo
campus, University of Namibia (UNAM). This field site is located near the centre of the CSSWs at
Oshakati, Namibia (17°40′49.02″S, 15°18′10.83″E).
Satellite remote sensing analyses
We estimated spatiotemporal variations in surface water-covered (flooded) areas (locally called“oshanas”) over north-central Namibia, including in the CSSWs. Two indices were used todetermine the surface water-covered area: the normalised difference water index (NDWI) and thenormalised difference polarisation index (NDPI).
The NDWI is defined by the following equation:𝜌𝜌𝑟𝑟𝑟𝑟𝑟𝑟 − 𝜌𝜌𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁 =𝜌𝜌𝑟𝑟𝑟𝑟𝑟𝑟 + 𝜌𝜌𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆
where ρred and ρSWIR are the reflectance of the red (0.6–0.7 μm) and shortwave infrared (1.4–3.0 μm)
electromagnetic spectrum bands (spectral regions), respectively.
The NDPI is defined as:
𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁 =36.5 𝑇𝑇𝑇𝑇 − 36.5 𝑇𝑇ℎ36.5 𝑇𝑇𝑇𝑇 + 36.5 𝑇𝑇ℎwhere 36.5Tv and 36.5Th are the brightness temperature (K) of the vertically and horizontally
polarised 36.5 GHz microwave, respectively (Sippel et al. 1994, Takeuchi et al. 2006).
The NDWI was calculated using MODIS visible and infrared band data at 250-m spatialresolution (16-day composite MOD13Q1 and MYD13Q1 data). The NDPI was determined fromAMSR-E microwave radiometer data at 25-km resolution. Both MODIS and AMSR-E are carriedon the Aqua satellite launched on 4 May 2002. Another MODIS is carried on the Terra satellite,
which was launched on 18 December 1999.
Firstly, by using MODIS data, we obtained a relationship of reflectance between the red andshortwave infrared spectral regions. Then, NDWI = –0.1 was determined as the threshold for
differentiation between a water surface and other land surfaces. On the basis of these analyses, theprobability of the existence of surface water was obtained during nine years (2003–2011; n = 414;
where n is the number of samples). However, because MODIS cannot observe land surface
conditions under cloud cover, a common condition during the wet season, obtaining a high temporal

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resolution (e.g. daily) NDWI mapping was difficult. Thus, the daily NDWI map was complemented
using the cloud-free NDPI map. Finally, we were able to estimate the daily surface water coverage
map from 2003 to 2011.
Bowen ratio-energy balance (BREB) measurement
We established two BREB systems (upper and lower sites) at the beginning of September 2012 in anexperimental sloped field within the Ogongo campus, UNAM. Both systems included an albedo meter(CHF-SRA01, Hukseflux, The Netherlands), a net radiometer (CPR-NR-LITE, Kipp & Zonen, TheNetherlands), two sets of air temperature-humidity sensors (CVS-HMP-155D, Vaisala, Finland), aheat flux plate (CHF-HFP-01, Campbell Scientific Inc., USA), three sets of soil temperature sensors(C-PTWP-10, Climatec Inc., Japan), three sets of water temperature sensors (C-PTWP-10, Climatec
Inc., Japan), three sets of soil moisture sensors (C-CS-616-7.5, Campbell Scientific Inc., USA), and awater level sensor (CWG-H10-C20-NGH, Climatec Inc., Japan). We additionally installed a windspeed-direction sensor (CYG-3002, Young, Japan) to one of the systems (upper slope site) and a windspeed sensor (CYG-3102, Young, Japan) to the other system (lower slope site).
Using the following equations, we determined sensible and latent heat fluxes at both sites in thesloped field:
𝑐𝑐𝑝𝑝 𝑇𝑇𝑇𝑇(𝑧𝑧1 ) − 𝑇𝑇𝑇𝑇(𝑧𝑧2 )𝐻𝐻𝛽𝛽 = ��=𝑙𝑙 𝑞𝑞(𝑧𝑧1 ) − 𝑞𝑞(𝑧𝑧2 )𝑙𝑙𝑙𝑙𝛽𝛽(𝑅𝑅𝑅𝑅 − 𝐺𝐺)
𝐻𝐻 =1 + 𝛽𝛽1(𝑅𝑅𝑅𝑅 − 𝐺𝐺)
𝑙𝑙𝑙𝑙 =
1 + 𝛽𝛽Here, Ta is air temperature, q is specific humidity, cp is the specific heat at constant pressure, l is thelatent heat for vaporization, Rn is the net radiation, G is the soil heat flux, H is the sensible heat flux,
lE is the latent heat flux, and β is the Bowen ratio. The subscripts 1 and 2 denote the different
measurement heights (z) of the air temperature-humidity sensors.
Several other environmental variables were also measured by using the BREB systems. The
main variables used in this study were downward shortwave radiation (Sd), upward shortwave
radiation (Su), surface albedo (α), soil water content (SWC), ground (soil) temperature (Tg), andprecipitation (Pr).

Fig. 1 Spatial distribution of the probability of the existence of surface water around the CSSWs obtainedusing the NDWI.

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RESULTS AND DISCUSSION
Seasonal changes in surface water coverage over the CSSWs
Figure 1 shows the spatial variation in the probability of the existence of surface water around theCSSWs obtained from the NDWI during 2003–2011 (n = 414). It should be noted that surface water
appeared in the lower (southern) part of the CSSWs around early January and propagated to theupper (northern) part of the CSSWs toward the middle of March. However, the origin of the surface
water remains uncertain. The water might have originated from a combination of rainfall, surface
runoff (flood water) from the Angolan Plateau flowing through seasonal rivers (oshanas), andgroundwater discharge.
The CSSWs include not only seasonal rivers (oshanas) but also small-scale wetlands(ondombes), which are patchily distributed over uplands (higher gap surfaces among seasonal rivers).
It is recommended that FDACS be introduced to ondombes (not oshanas) because local farmers caneasily utilise ondombes for their own productive activities. The spatial scale of ondombes is mostly
less than 500 m. Thus, because of the limited spatial resolution of MODIS, the probability
distribution of water-covered areas (Fig. 1) mainly shows the existence of surface water in seasonalrivers (oshanas). Further study of the coincidence of surface water between oshanas and ondombes
is needed.
Our field observations to date suggest that the surface (pond) water of ondombes originatesfrom rainfall and/or groundwater, not from the overland surface flow of oshanas. For more precise
water budget evaluations, it is necessary to clarify the spatial distribution of rainfall as well as thatof groundwater levels (and/or the groundwater characteristics of ondombes and the surface waterfeatures of oshanas). Monitoring of the spatiotemporal distributions of rainfall, surface runoff,evapotranspiration, and groundwater depth is recommended.
We have established BREB systems in the experimental sloped field at Ogongo campus ofUNAM, allowing for evaluation of the difference in evapotranspiration between pearl milletdominated crop fields and rice-dominated crop fields after the introduction of FDACS. This topic isevaluated in the next section.
Effect on evapotranspiration of introducing a mixed cropping system
Figure 2 shows time series (from mid-September 2012 to mid-April 2013) in air temperature (Ta),vapour pressure deficit (VPD), wind speed (Ws), daytime evapotranspiration (ET), and precipitation(Pr) observed at the upper (millet-dominated) and lower (rice-dominated) sites of the sloped
experimental field of Ogongo campus, UNAM. There are no significant differences in Ta, VPD, andWs between the upper and lower sites. At both sites, Ta and VPD decreased after the rainy season
began. After flooding of the lower field, evapotranspiration (ET) at this field became higher thanthat at the upper site. After the rainy season began, ET at the lower site became small, mainly becauseof the decrease in Ta and VPD. After planting rice and pearl millet, ET at the lower (rice) site
increased again probably due to the contribution from rice cropping.
Very little rain fell during the 2012–2013 wet season around the CSSWs. Even pearl millet, adrought-tolerant crop, suffered in this severe drought. Crops at the upper site of the experimental
sloped field failed during this drought. Thus, the upper site had almost no vegetation and wascovered by bare soil surface. In contrast, the lower site was irrigated artificially, and the rice cropgrew successfully. These surface conditions affected the energy (radiation and heat) budget in thefields.
Figure 3 shows time series of net radiation (Rn), sensible heat flux (H), latent heat flux (lE), andsurface albedo (α) together with Pr at both sites. After flooding at the lower site, both Rn and lE
became higher at the lower site than at the upper site. On the other hand, α and H became higher atthe upper site. These radiation and heat budgets were controlled mainly by water and vegetation
conditions. Thus, the effects of water level, soil moisture, and vegetation densities on the surface
energy budget as well as the ET characteristics should be analysed in the near future.

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Ta(℃)

35

● upper site
● lower site

30
25
200

1.4
1.2
10.8
0.6
0.4
0.2
0650

540ET (mm hour-1)

VPD (hPa)

504030204Pr(mm)-1Ws (m s )10323020101-100 -80 -60 -40 -20 0202012
DOY4060 80
2013

100 120020-100 -80 -60 -40 -20 02012
DOY4060 80
2013

100 12060 80
2013

100 120Fig. 2 Time series of air temperature (Ta), vapour pressure deficit (VPD), wind speed (Ws), daytime
evapotranspiration (ET), and precipitation (Pr) at the upper and lower sites of the sloped experimental
field at Ogongo campus, University of Namibia (UNAM). DOY is the day of the year 2013. (DOY 1means 1 January 2013.)
800Rn (W m-2)

700
600

● upper site
● lower site

500
400
300
200

0.50.4600

α400

0.30.2200

0.10800

050

600

40Pr(mm)-2lE (W m )-2H (W m )

800

400
200
0-100 -80 -60 -40 -20 020
2012
DOY3020104060 80
2013

100 1200-100 -80 -60 -40 -20 0202012
DOY

40

Fig. 3 Same as Fig. 2 but for net radiation (Rn), sensible heat flux (H), latent heat flux (lE), surface albedo(α), and precipitation (Pr).

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CONCLUSIONSThe CSSWs have the highest human population densities in Namibia (50–100 people/km2). The
small wetlands (ondombes) are used partly for pastoralism and also for numerous individual family
settlements and townships. This study is the first attempt to evaluate the water dynamics for water
security in the case of the introduction of rice-millet mixed-cropping systems (FDACS) to ondombes
in the CSSWs in north-central Namibia. The purposes of this study were: (1) to understand surface
flooding water around the CSSWs, and (2) to assess the effects on evapotranspiration of introducing
mixed-cropping systems in the region.
Two main results relating to these objectives were obtained:(1) Regarding seasonal changes in hydrology, analysis of MODIS and AMSR-E satellite remotesensing data revealed that surface water first begins to appear in the southern (lower) part of theCSSWs.
(2) Data obtained by two BREB systems, which were established in September 2012 in anexperimental research field at Ogongo campus of the UNAM, showed the importance of waterand vegetation conditions for the introduction of FDACS to the ondombes.
Acknowledgements This study was supported by an international research/development (R/D)
project of the Science and Technology Research Partnership for Sustainable Development(SATREPS) entitled “Flood- and drought-adaptive cropping systems to conserve water
environments in semi-arid regions” (P.I.: M. Iijima). We thank the Japan Science and Technology
Agency (JST) and the Japan International Cooperation Agency (JICA) for their kind coordination.
We also thank all members of the Faculty of Agriculture and Natural Resources, University ofNamibia (UNAM), for kindly collaborating with our research.
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