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Research Project: Understanding Water-Driven Ecohydrologic and Erosion Processes in the Semiarid Southwest to Improve Watershed Management

Location: Southwest Watershed Research Center

Title: Implementing dynamic root optimization in Noah-MP for simulating phreatophytic root water uptake

Author
item Wang, P. - Chinese Academy Of Sciences
item Niu, G.y. - University Of Arizona
item Fang, Y.h. - University Of Arizona
item Wu, R.j. - University Of Arizona
item Yu, J.j. - Chinese Academy Of Sciences
item Yuan, G.f. - Chinese Academy Of Sciences
item Pozdniakov, S.p. - Moscow State University
item Scott, Russell - Russ

Submitted to: Water Resources Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/20/2018
Publication Date: 4/18/2018
Citation: Wang, P., Niu, G., Fang, Y., Wu, R., Yu, J., Yuan, G., Pozdniakov, S., Scott, R.L. 2018. Implementing dynamic root optimization in Noah-MP for simulating phreatophytic root water uptake. Water Resources Research. 54:1560-1575.
DOI: https://doi.org/10.1002/2017WR021061

Interpretive Summary: Plants are known to adjust their root systems to adapt to changing soil water conditions. However, most current land surface models use a prescribed, static root profile. We implemented a new representation of root dynamics into a commonly used land surface model to allow the modeled root profile to optimally respond to changing soil moisture conditions. We tested the model against observations in a riparian forest under a hyper-arid climate in northwestern China. Compared to the prescribed static root profile, the dynamic root model largely improved the simulation of surface energy and water fluxes, particularly during the growing seasons. At this site, the tree water use is mainly supplied by groundwater. The land surface model with the improved root dynamics provides a useful tool for understanding the responses of riparian ecosystems to climate change and groundwater extraction in arid and semi-arid regions.

Technical Abstract: Plants are known to adjust their root systems to adapt to changing subsurface water conditions. However, most current land surface models (LSMs) use a prescribed, static root profile, which cuts off the interactions between soil moisture and root dynamics. In this paper, we implemented an optimality-based model of root dynamics into the Noah-MP LSM. The dynamic root model numerically describes the natural optimization of the root profile in response to changes in the soil moisture profile to maintain a sufficient amount of water in plant tissues to meet the water demand for transpiration. Considering direct root water uptake from groundwater, we introduced a “watered” root layer into Noah-MP to represent the overall root density within the saturated zone. We tested the model against observations in a riparian Tamarix spp. stand under a hyper-arid climate (~35 mm/yr precipitation) in northwestern China. Compared to the prescribed static root profile in Noah-MP, the dynamic root model largely improved the simulation of surface energy and water fluxes, particularly during the growing seasons. At this site, the transpiration (~478 mm/yr) is mainly (~60%) supplied by groundwater recharged by lateral flow through naturally optimized roots. Noah-MP with the improved root dynamics provides a useful tool for understanding the responses of riparian ecosystems to climate change and groundwater extraction in arid and semi-arid regions. Future studies should focus on how plants optimize the allocation of assimilated carbon to root biomass to maximize photosynthesis, and the modeling system should be tested over a wide range of environments.