Impacts of Noah model physics on catchment‐scale runoff simulations

Abstract

Noah model physics options validated for the source region of the Yellow River (SRYR) are applied to investigate their ability in reproducing runoff at the catchment scale. Three sets of augmentations are implemented affecting descriptions of (i) turbulent and soil heat transport (Noah‐H), (ii) soil water flow (Noah‐W), and (iii) frozen ground processes (Noah‐F). Five numerical experiments are designed with the three augmented versions, a control run with default model physics and a run with all augmentations (Noah‐A). Each experiment is set up with vegetation and soil parameters from Weather Research and Forecasting data set, soil organic matter content from China Soil Database, 0.1° atmospheric forcing data from Institute of Tibetan Plateau Research (Chinese Academy of Sciences), and initial equilibrium model states achieved using a single‐year recurrent spin‐up. In situ heat flux, soil temperature (Ts), and soil moisture (θ) profile measurements are available for point‐scale assessment, whereas monthly streamflow is utilized for the catchment‐scale evaluation. The comparison with point measurements shows that the augmentations invoked with Noah‐H resolve issues with the heat flux and Ts simulation and Noah‐W mitigates deficiencies in the θ simulation, while Noah‐A yields improvements for both simulated surface energy and water budgets. In contrast, Noah‐F has a minor effect. Also, at catchment scale, the best model performance is found for Noah‐A leading to a base flow‐dominated runoff regime, whereby the surface runoff contribution remains significant. This study highlights the need for a complete description of vertical heat and water exchanges to correctly simulate the runoff in the seasonally frozen and high‐altitude SRYR at the catchment scale.

Assessment of Noah land surface model with various runoff parameterizations over a Tibetan river

“…As in Zheng et al . [], the monthly streamflow (m 3 ) measurements (section 3.1) are converted for validation to the monthly area‐averaged total runoff ( R , mm) by dividing by the SRYR total area (km 2 ), and the surface runoff ( R s ) and subsurface runoff ( R b ) simulated with the Noah LSM are accumulated to produce the monthly area‐averaged R . Additionally, the heat flux and profile soil temperature and moisture simulations are extracted from the grid elements where the Maduo and Maqu stations are located for the comparison with those in situ measurements.…”

“…Surface runoff ( R s , in m s −1 ) in the Noah LSM consists of infiltration excess runoff from the unfrozen part of the model grid and direct runoff from the impermeable frozen part ( f imp , ‐) [ Schaake et al ., ; Koren et al ., ]: where Δ t is the time step (s), W d is the soil water deficit within the soil column (m), P x is the precipitation falling on the ground (m), K dt is an empirical parameter taken as 3.0 d −1 , and the impermeable frozen fraction is as in Zheng et al . [] assuming a gamma probability distribution for the soil ice content ( W ice , m) within root zone: where Δ z is the soil depth (m), n root is the number of soil layers within the root zone (‐), θ ice is the soil ice content (m 3 m −3 ), W cr is the critical ice content above which the frozen ground is impermeable taken as 0.15 m, α is a shape parameter of the gamma probability distribution (‐) taken as 3, and v is the upper limit of an integral function representing the spatial variability of frozen depth [ Koren et al ., ].…”

“…These in situ measurements are available from November 2009 to December 2010, and detailed information about the measurements and data processing is given in Zheng et al . [].…”

“…Recently, Zheng et al . [] incorporated the above findings into the Noah LSM and illustrated that a complete treatment of both warm and cold season hydrometeorological processes is imperative for a correct simulation of the runoff regime over a seasonally frozen Tibetan river. However, the Noah LSM ignores the interactions between soil water and groundwater as well as the topography‐driven lateral surface and subsurface flow that impact the runoff.…”

“…The version of the Noah LSM described in Zheng et al . [] is adopted and modified to include the three selected runoff parameterizations which are currently adopted by (i) Noah‐MP [ Niu et al ., ], (ii) CLM [ Oleson et al ., ], and (iii) CLM‐VIC [ Li et al ., ].…”