ARCHIVED CONTENT: In December 2020, the CZO program was succeeded by the Critical Zone Collaborative Network (CZ Net) ×

Joint HWR/SNRE Seminar


What happens to the rain? Revisiting Penman's (1961) fundamental question in hydrology within the context of western water resources

Harshbarger 206

Presentation by Dr. Paul D. Brooks, Professor, HWRS, and Director, Center for Sustainability of semi-Arid Hydrology and Riparian Areas (SAHRA).


Concurrent changes in climate, land cover, and population growth present major challenges to the management of natural resources worldwide. Pronounced warming and widespread vegetation change in western North America are prime examples of ongoing perturbations that complicate water resource management by pushing coupled human and natural systems outside of the observed range of natural variability.  Addressing these challenges requires simultaneous efforts on three questions:

1) How has climate forcing (e.g. temperature, precipitation, radiation) varied in the past and how will it change in the future?

2) How will these changes affect the partitioning of precipitation into water resources for society or ecosystems? and

3) How can management promote the sustainable use of limited water supplies? Considerable progress has been made over the last 10-15 years on each of these questions, but perhaps the least well understood is question 2, not the least because, like politics, partitioning is local.

This presentation will present recent work on understanding of how changes in climate and vegetation interact with landforms to control the partitioning of precipitation (P) into available water resources in the western US.  Although river discharge (Q) provides societally relevant and spatially extensive data on one aspect of hydrologic partitioning, small (10-15%) runoff ratios (Q:P) provide limited insight into how the remaining 90% of precipitation is partitioned into groundwater recharge (dS), evaporation (E), and transpiration (T).

Historically, it has been rare to have simultaneous observations of all components of the water balance, often leading to the implicit assumption that the response of E, T, and dS to changes in climate or landcover can be predicted from past relationships.

Moving beyond this assumption of stationarity has been identified as a primary challenge in water resource management. To address this challenge, the NSF Science and Technology Center SAHRA, the NSF Jemez-Santa Catalina Criticall Zone Observatory, and several related research projects have focused on quantifying hydrologic partitioning using colocated observations, integrated modeling, and distributed data collection throughout the western US.

Bringing together physical hydrology, hydrochemistry, ecohydrology, hydrometeorology, and biogeochemistry, these projects use multiple lines of evidence to revisit a fundamental question in hydrology, perhaps better rephrased as “where does precipitation go and how does it get there?” The emerging answers to this question identify when, where, and which water resources are at risk, and inform local and regional water resource and landscape management decisions.

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