Abstract:The current generation of hydrological models has been widely criticized for their inability to adequately simulate hydrological processes. In this study, we evaluate competing model representations of hydrological processes with respect to their capability to simulate observed processes in the Mahurangi River basin in Northland, New Zealand. In the first part of this two-part series, the precipitation, soil moisture, and flow data in the Mahurangi were used to estimate the dominant hydrological processes and explore several options for their suitable mathematical representation. In this paper, diagnostic tests are applied to gain several insights for model selection. The analysis highlights dominant hydrological processes (e.g. the importance of vertical drainage and baseflow compared to sub-surface stormflow), provides guidance for the choice of modelling approaches (e.g. implicitly representing sub-grid heterogeneity in soils), and helps infer appropriate values for model parameters. The approach used in this paper demonstrates the benefits of flexible model structures in the context of hypothesis testing, in particular, supporting a more systematic exploration of current ambiguities in hydrological process representation. The challenge for the hydrological community is to make better use of the available data, not only to estimate parameter values but also to diagnostically identify more scientifically defensible model structures.

From rainfall interception at the canopy to added soil cohesion within the roots, plants play a significant role in directing local geomorphic dynamics, and vice versa. The consequences at the regional scale, however, are less well known despite being the subject of conjecture spanning three centuries. In light of this, the numerical model CHILD is equipped with coupled vegetation-erosion dynamics, allowing for sensitivity analysis on the various aspects of vegetation behavior. The processes considered are plant growth, plant death, and the additional resistance imparted by plants against erosion. With each process is associated a single parameter, whose effects on the spatio-temporal nature of a fluvially-dominated 1.8 by 1.8 km landscape is studied.While each parameter possessed its own geomorphic signature, some common effects were shared by all, and thus were essentially a result of the vegetation itself. Through their inhibition of erosion, plants steepened the topography and made erosive events more extreme, yet became established more widely throughout the area. As a result, erosion rates varied spatially, leading to periodic stream capture and oscillating mean elevation, erosion rates, and vegetation density, and a meta-stable biophysiography. Despite having stationary, yet stochastic, climatic forcing, and steady uplift, this oscillatory behavior arises out of the meta-stability of the vegetation-erosion coupling itself. This has implications for the nature of cut-fill cycles, and the stability and diversity of vegetation-mantled landscapes. Thanks also to the other group members Frederic Chagnon, Jean Fitzmaurice, Valeri Ivanov, Scott Rybarczyk and Enrique Vivoni-Gallart. Cross-campus, and crosscontinent, helpful discussions and suggestions were provided by

[1] Root depth and distribution are vital components of a plant's strategy for growth and survival in water-limited ecosystems and play significant roles in hydrologic and biogeochemical cycling. Knowledge of root profiles is invaluable in measuring and predicting ecosystem dynamics, yet data on root profiles are difficult to obtain. We developed an ecohydrological model of environmental forcing, soil moisture dynamics, and transpiration to explore dependencies of optimal rooting on edaphic, climatic, and physiological factors in water-limited ecosystems. The analysis considers individual plants with fixed biomass. Results of the optimization approach are consistent with profiles observed in nature. Optimal rooting was progressively deeper, moving from clay to loam, silt and then sand, and in wetter and cooler environments. Climates with the majority of the rainfall in winter produced deeper roots than if the rain fell in summer. Long and infrequent storms also favored deeper rooting. Plants that exhibit water stress at slight soil moisture deficiencies consistently showed deeper optimal root profiles. Silt generated the greatest sensitivity to differences in climatic and physiological parameters. The depth of rooting is governed by the depth to which water infiltrates, as influenced by soil properties and the timing and magnitude of water input and evaporative demand. These results provide a mechanistic illustration of the diversity of rooting strategies in nature.

New communication and digital image technologies have enabled the public to produce large quantities of flood observations and share them through social media. In addition to flood incident reports, valuable hydraulic data such as the extent and depths of inundated areas and flow rate estimates can be computed using messages, photos and videos produced by citizens. Such crowdsourced data help improve the understanding and modelling of flood hazard. Since little feedback on similar initiatives is available, we introduce three recent citizen science projects which have been launched independently by research organisations to quantitatively document flood flows in catchments and urban areas of Argentina, France, and New Zealand. Key drivers for success appear to be: a clear and simple procedure, suitable tools for data collecting and processing, an efficient communication plan, the support of local stakeholders, and the public awareness of natural hazards.

[1] Drainage density has long been observed to vary among climates, a relationship often attributable to differences in the erosivity of runoff and resistivity of vegetation. There is also evidence, though much less, that relief and channel concavity also vary with climate. The biophysical chain of events that connect climate to these topographic expressions, however, deserves greater attention. Using a numerical landscape evolution model, we examine how a gradient of mean annual precipitation is expressed in the topography of low-to medium-relief, sediment-mantled, water-limited ecosystems. We find equilibrium landscapes have the lowest drainage density, greatest relief, and lowest concavity at intermediate levels of rainfall. This climatic threshold represents the transition from vegetation-dominated sediment flux in drier climates to runoff-dominated flux with more precipitation. In drier climates, marginal increases in precipitation manifest themselves primarily as increases in vegetation while runoff is relatively constant; this acts to suppress sediment transport and decreases the drainage density. In wetter environments, marginal increases in precipitation lead to increases primarily in runoff while vegetation cover remains relatively constant; this results in increased sediment transport and increases the drainage density. The location of the transition depends at least in part on plant community structure and composition. The modeling study illustrates the complexities inherent in biogeomorphic systems but also that a simplified conceptual model of landscape evolution may indeed be sufficient to understand the large-scale patterns. The study also illustrates the opportunities offered by approaching questions of landform development from an ecohydrological perspective.Citation: Collins, D. B. G., and R. L. Bras (2010), Climatic and ecological controls of equilibrium drainage density, relief, and channel concavity in dry lands, Water Resour. Res., 46, W04508,