II. The Drainage Basin Model

The issue of water availability and climate change are obviously important linked controls on terrestrial ecosystems; therefore, in this Report we consider separately the work on the terrestrial water cycle even though it is coupled to the work on the Terrestrial Ecosystem Model (TEM).

Several ongoing activities have supported progress toward improvement of the Drainage Basin Model (DBM) component of our IDS effort. In the last 12 months we have focused upon two efforts:

• Refinement of the Water Balance Model (WBM), and
• Development of required data bases.

We have reported on this work not only the scientific journals but also through contributions to national and international workshops.

II. 1. The Next step in refining our Water Balance Model: An Inter-comparison of Evapotranspiration methods in support of NPP modeling. Closing the water cycle is an essential element in developing biogeochemical, prognostic models that can couple to models of the climate subsystem; thereby, providing Earth system models.

Starting with a paper published in Water Resources Research in 1996 (Federer et al.), we have been carefully analyzing the reliability of potential evaporation functional structures. Potential evaporation is an important precursor to plant-atmosphere CO₂ flux calculations commonly employed in global-scale terrestrial net primary productivity (NPP) models and thus is of direct relevance to our IDS research and the EOS mission.

Eleven potential evaporation functions were tested, ranging from simple temperature-driven equations to physically-based combination approaches. Both reference surface and land cover-dependent methods were evaluated using a priori parameter assignments. The potential evaporation functional structures were incorporated into our global-scale Water Balance Model (WBM) tested over the conterminous United States (which we note is the VEMAP domain) using mean monthly climatic drivers and other biophysical inputs at 0.5 degree grid spatial scale. For each potential evaporation structural form water budgets were computed on 3,265 individual grid cells using a quasi-monthly time step. For 679 locations with reliable discharge measurements and grid-based precipitation, we compared simulated to measured annual and monthly runoff. Our objective measure of potential evaporation functions performance was mean bias in simulated evapotranspiration relative to observation, defined as the mean difference between potential evaporation functions and grid-based precipitation minus measured streamflow.

Mean bias for individual methods ranged from -94 to +119 mm y^-1 for reference surface methods and -50 to +38 mm y^-1 for land cover-dependent methods.

What are the potential consequences of such errors in water balance closure to the accuracy of NPP estimates?

In tests using our Terrestrial Ecosystem Model (TEM) across the US, the range in evapotranspiration bias arising from alternative potential evaporation structural forms yielded a range in NPP response from 400 to 475 g C m^-2 y^-1. This range is nearly identical to that found in the recent NPP model inter-comparison study, VEMAP. The generality of this finding, however, has yet to be tested at the global scale. Our findings also suggest that the calibration and validation of NPP models be conditioned upon comparison of water budget calculations to suitable records of observed discharge.

II. 2. The Next Steps in Developing and Distributing data for Water Balance Models. In 1996, we published the first version of our EOS-DIS River Discharge Database (RivDIS v1.0), as part of the UNESCO Technical Documents in Hydrology series. Time series data are presented on nearly 1000 rivers worldwide, providing the community with important calibration / validation targets for models as well as important information for planning field campaigns (i.e., LBA). This data is part of the larger Global Hydrological Archive and Analysis System (GHAAS) developed as part of our EOS-DIS effort.

To facilitate use of this information, we will provide to interested parties the data on diskette or CD-ROM. For those with access to the Internet, the data sets will soon be available for pre-visualization and downloading from our EOS World Wide Web site at the University of New Hampshire. We hope that our EOS-DIS system will have GIS capability over the web.¹³ We are working with the Oak Ridge DAAC which plans to archive and disseminate this information as part of its 1997 activities. The National Snow and Ice Data Center will be supporting the distribution of a pan-Arctic version of RivDIS v1.0 supplemented with WMO-ACSYS data.

Global DBM River Networks. The Simulated Topological Network at 30-minute resolution (STN-30) version 4.2 is now complete. It is similar to earlier versions of the STN-30 but has the important feature of being geographically co-registered to two important data bases: a) the RivDIS (v1.0), and, b) for the purposes of river biogeochemistry analysis, M. Meybeck's Global River Inputs to the Ocean (GEMS-Water-GLORI) chemical constituent data base.

With a geo-specific characterization of both water and chemical fluxes, we provide to the community the necessary data to assess with high spatial resolution the linkages between the terrestrial biosphere and the oceans.

Global Reservoir Database. As part of our larger theme of exploring anthropogenic changes to biogeochemical cycles, we are attempting to consider explicitly the widespread alteration of natural river systems arising from water engineering works. One particularly important such alteration is the substantial increase in standing stock of water resident in river systems due to impoundment. Not only does large-scale impoundment have impacts on river chemistry, sediment and carbon flux, but it is also important in defining regional water balance and the character of hydrographs. Corrections must thus be made to account for their impacts when constructing calibration / validation targets for our WBM and DBM simulations. Moreover, this data will be valuable not only to the global change community but to operational users and scientist concerned with natural hazards.

In a first study to be published in AMBIO in May, 1997 we analyzed the scope and potential impact of reservoir construction on the global network of rivers. Storage behind dams represents an eight-fold increase in the standing stock of river water, with residence times for individual impoundments spanning less than one day to several years. The mouths of several of the world's largest rivers show a reservoir-induced aging of runoff in excess of three months. Globally, the mean age of river water has likely tripled to well over one month. The imprint of such storage persists downstream, leading to significant changes in net water balance, flow regime, re-oxygenation of surface waters, and sediment transport.

For example, our analysis suggests that the global impact of such changes on suspended sediment and potential carbon flux could be very large. We estimated that more than 40% of global river discharge is intercepted by large impoundments and that a significant proportion (70%) of this river flow maintains a theoretical trapping efficiency in excess of 50%. We conclude that the pandemic construction of large dams and other engineering works now represents an important component of the terrestrial water cycle and one that merits appropriate consideration in future global change studies.

Remotely-sensed fluvial dynamics. We are exploring the use of Nimbus-7 passive microwave imagery to infer the dynamics of large tropical floodplain rivers. This sets the stage for work with the PM-1 passive microwave data. Our first series of tests has been on the Amazon River (published in Water Resources Research in 1996) in which we used a series of straightforward statistical models linking 37 GHz polarization temperature differences to calibrated discharges along more than 1400 km of mainstem river documenting the opportunities and limits of passive microwave. An El Nino signal was clearly evident in both observed discharge records and from the satellite-based monitoring.

Work¹⁴ continues with J. Melack (University of California, Santa Barbara), of the EOS-RAM IDS Team, to obtain more accurate floodplain inundation estimates over a multi-year period.

II. 3. Participation in National and International Fora. Despite its enormous importance to human society, an arguably neglected element of the global change debate surrounds linkages between the continental land mass, river systems, and the coastal /nearshore environment. We continue to view this as a poorly quantified yet potentially significant Earth System question.

Co-investigator Vorosmarty convened a workshop in December of 1994 on behalf of IGBP with the aim of developing a coherent modeling strategy to address this issue. Results of this workshop and recommendations for IGBP synthesis are currently being published as an IGBP report. He also participated as an invited keynote speaker on this subject in the Fourth IGBP-Scientific Advisory Committee meeting in Beijing (November 1995) which is being published as a contribution to a Cambridge University Press book on Asian and Global Change. A strategy paper written by co-investigators Vorosmarty and Peterson on fluvial transport is also being published as a contribution to the US-SCOPE Committee volume on estuarine synthesis. Co-investigator Vorosmarty also served as a co-Chair of the hydrology sub-group at the LBA Planning meeting in Sao Jose dos Campos, BRAZIL during June 1996.

Co-investigator Vorosmarty continued during 1996 as a member of the Scientific Steering Committee of the BAHC (Biospheric Aspects of the Hydrological Cycles) Core Project, involved specifically with Focus 3 Activities that examine the diversity of biosphere atmosphere interactions over a spectrum of space and time scales. He is a member in standing of the PAGES Fluvial Systems Working Group. He is also serves as Secretary of the IAHS International Commission on Atmosphere-Soil-Vegetation Relations. Co-investigator Vorosmarty is a member of the Oak Ridge DAAC User Working Group.