Anne Kremer Huth and Roger Bales install scour chains in a gravel bar of the Sab Pedro

The major goal of Thrust Area 3 (TA3) is to understand how riparian systems affect and are affected by changes in water quantity (e.g., due to ground-water pumping or conjunctive stream-water management) or water quality (e.g., nitrogen inputs from precipitation or agricultural runoff).

Research in TA3 is generally separated into two foci:

1. Water and carbon exchange in riparian systems.
2. Nutrient cycling in riparian systems.

These two groups are linked through the common need to understand the dynamics of the water balance and
movement within the riparian zone, as well as within
the basin.

This need for common knowledge also necessitates the collaboration of TA3 members with researchers from TA1, TA2 and TA4 for understanding the possible long-term effects of climate-change and anthropogenic stressors on the health of the riparian system. Figure 3.1 illustrates the stratification within TA3, the links within and across the SAHRA TAs, and the collaboration with other research groups with which there are strong active partnerships. Much of the research being performed is a continuation of the SALSA (Semi-Arid Land Surface Atmosphere) project, and is heavily leveraged with many entities in the Upper San Pedro River Valley.

3.1 Water and Carbon Exchange in Riparian Systems
The major goal of this group is to develop a holistic approach for assessing the effects of ground-water development and conjunctive surface-water management on riparian ecosystems. Most of the historic riparian areas in the southwest have disappeared over the last century. This is believed to be a direct consequence of groundwater pumping for agriculture, mining, and municipal needs. These riparian corridors harbor a large majority of the regional biodiversity, and provide an aesthetic value for many humans, and presently, they are facing great stress. Researchers in this group seek to understand the effects of changing hydrologic dynamics (ground, vadose, surface, and plant water movements) on riparian ecology on the time scales of hours to years.

Individual research efforts include:

• Determination of the interplay of the riparian plants and the movement and availability of water, and how the water usage is related to nutrient storage and fluxes.
  Dave Williams (UA), Russ Scott (UA), Guanghui Lin (Biosphere 2)
  
  
• Response of riparian vegetation water uptake under stress due to groundwater pumping.
  Paul Ferré, Tom Maddock (UA)
  
  
• Relationship between hydrologic and geomorphologic conditions and survival rate of exotic versus native riparian tree seedlings.
  Julie Stromberg (ASU)
  
  
• What are the hydrologic, ecological, and anthropogenic controls on spatial and temporal distribution of flowing and non-flowing reaches of desert streams?
  Stan Leake (USGS)

The collaborative relationships both within this subtask and with other projects are illustrated in left side of Figure 3.1. These projects are closely related as they are focused directly on the interplay of the riparian plants with the movement and availability of water. The sum of these projects will be new tools to predict ecological succession with changing hydrologic stress. Since this work is interested in long-term effects of hydrologic changes on the riparian ecology, there are connections with the infiltration/runoff studies in TA1 and the basin-scale, water-balance studies in TA2. In addition, the micrometeorological flux measurements will tie into similar measurements in TA1 and TA2. This data will also be used to calibrate land-surface models in TA4. Furthermore, the understanding of what sources of water are used by which plant communities will be critical information for basin management, and therefore will tie into TA5.

Activities and plans for next reporting period
The major activities of the various task areas for the last year are listed below:

3.2 Nutrient Cycling in Riparian Systems
The central objective of this group is to understand the role of biogeochemical cycling in the hyporheic zones of streams as a pathway for removal of natural and anthropogenic solutes in riparian ecosystems. Riparian zones can be highly variable, depending on channel type, perturbations, and local vegetation. The interaction of nutrients and biota affects redox conditions in shallow ground water, which influences the movement of anthropogenic inputs in the system (e.g., metals). The efficiency of this biogeochemical cycling depends on availability of nutrients, characteristics of the riparian ecotone, and hydrologic characteristics of hyporheic flow paths. In streams of the semi-arid Southwest, the common limiting nutrient is nitrogen. Large nitrogen inputs from precipitation, fertilizer applications, and other human activities thus could potentially contribute to eutrophication of aquatic ecosystems. In addition, rainfall and nitrogen inputs from floods exhibit seasonal patterns.

Individual research efforts include:

• Understanding the links between hydrology, geomorphology and nutrient cycling within the semi-arid riparian/stream system.
  Grimm (ASU), Conklin (UA), Villinski (UA)



• Contribution of terrestrially produced dissolved organic matter (DOM) to carbon and nitrogen cycling in semi-arid riparian ecosystems.
  Brooks (UA)



• The influence of episodic flow events on nutrient and sediment loads in semi-arid streams.
  Conklin, Villinski (UA-HWR)



• Relating hydrologic flowpaths to nutrient speciation and retention.
  Grimm, Schade, Lewis (ASU)



• Spatial and temporal variability of the sources, sinks and speciation of nitrogen and carbon in semi-arid rivers.
  Villinski, Hogan, Brooks, Conklin (UA-HWR)

All are concerned with nutrient cycling in the riparian/stream system. Crucial to all these projects and those of the other focus of TA3 is the understanding of the flow of water in the various hydrologic units within the system. Therefore, these projects will not only share data within TA3, but will also share data with TA1 and TA2 regarding hydrologic inputs into the riparian system. The ultimate goal is to create a "nutrient" model that combined with the other modeling activities within the San Pedro River Basin can result in an integrated riparian ecosystem model.

Relevance of Thrust Area efforts to SAHRA goals
The results from these research activities will be included in the integrated modeling effort of TA4, adding the critical nutrient component that is often overlooked in hydrologic modeling.