Hydrologic Memory Research Articles

Propagation of soil moisture memory to streamflow and evapotranspiration in Europe

Abstract. As a key variable of the land-climate system soil moisture is a main driver of streamflow and evapotranspiration under certain conditions. Soil moisture furthermore exhibits outstanding memory (persistence) characteristics. Many studies also report distinct low frequency variations for streamflow, which are likely related to soil moisture memory. Using data from over 100 near-natural catchments located across Europe, we investigate in this study the connection between soil moisture memory and the respective memory of streamflow and evapotranspiration on different time scales. For this purpose we use a simple water balance model in which dependencies of runoff (normalised by precipitation) and evapotranspiration (normalised by radiation) on soil moisture are fitted using streamflow observations. The model therefore allows us to compute the memory characteristics of soil moisture, streamflow and evapotranspiration on the catchment scale. We find considerable memory in soil moisture and streamflow in many parts of the continent, and evapotranspiration also displays some memory at monthly time scale in some catchments. We show that the memory of streamflow and evapotranspiration jointly depend on soil moisture memory and on the strength of the coupling of streamflow and evapotranspiration to soil moisture. Furthermore, we find that the coupling strengths of streamflow and evapotranspiration to soil moisture depend on the shape of the fitted dependencies and on the variance of the meteorological forcing. To better interpret the magnitude of the respective memories across Europe, we finally provide a new perspective on hydrological memory by relating it to the mean duration required to recover from anomalies exceeding a certain threshold.

Land cover effects on runoff patterns in eastern Piedmont (USA) watersheds

Physiography and land cover determine the hydrologic response of watersheds to climatic events. However, vast differences in climate regimes and variation of landscape attributes among watersheds (including size) have prevented the establishment of general relationships between land cover and runoff patterns across broad scales. This paper addresses these difficulties by using power spectral analysis to characterize area-normalized runoff patterns and then compare these patterns with landscape features among watersheds within the same physiographic region. We assembled long-term precipitation and runoff data for 87 watersheds (first to seventh order) within the eastern Piedmont (USA) that contained a wide variety of land cover types, collected environmental data for each watershed, and compared the datasets using a variety of statistical measures. The effect of land cover on runoff patterns was confirmed. Urban-dominated watersheds were flashier and had less hydrologic memory compared with forest-dominated watersheds, whereas watersheds with high wetland coverage had greater hydrologic memory. We also detected a 10–15% urban threshold above which urban coverage became the dominant control on runoff patterns. When spectral properties of runoff were compared across stream orders, a threshold after the third order was detected at which watershed processes became dominant over precipitation regime in determining runoff patterns. Finally, we present a matrix that characterizes the hydrologic signatures of rivers based on precipitation versus landscape effects and low-frequency versus high-frequency events. The concepts and methods presented can be generally applied to all river systems to characterize multiscale patterns of watershed runoff. Copyright © 2013 John Wiley & Sons, Ltd.

Effect of water table dynamics on land surface hydrologic memory

The representation of groundwater dynamics in land surface models has received considerable attention in recent years. Most studies have found that soil moisture increases after adding a groundwater component because of the additional supply of water to the root zone. However, the effect of groundwater on land surface hydrologic memory (persistence) has not been explored thoroughly. In this study we investigate the effect of water table dynamics on National Center for Atmospheric Research Community Land Model hydrologic simulations in terms of land surface hydrologic memory. Unlike soil water or evapotranspiration, results show that land surface hydrologic memory does not always increase after adding a groundwater component. In regions where the water table level is intermediate, land surface hydrologic memory can even decrease, which occurs when soil moisture and capillary rise from groundwater are not in phase with each other. Further, we explore the hypothesis that in addition to atmospheric forcing, groundwater variations may also play an important role in affecting land surface hydrologic memory. Analyses show that feedbacks of groundwater on land surface hydrologic memory can be positive, negative, or neutral, depending on water table dynamics. In regions where the water table is shallow, the damping process of soil moisture variations by groundwater is not significant, and soil moisture variations are mostly controlled by random noise from atmospheric forcing. In contrast, in regions where the water table is very deep, capillary fluxes from groundwater are small, having limited potential to affect soil moisture variations. Therefore, a positive feedback of groundwater to land surface hydrologic memory is observed in a transition zone between deep and shallow water tables, where capillary fluxes act as a buffer by reducing high‐frequency soil moisture variations resulting in longer land surface hydrologic memory.

Quantifying urbanization-associated changes in terrestrial hydrologic system memory

A common feature of watershed urbanization is increased hydrograph ‘flashiness,’ whereby river discharge fluctuations grow more erratic. Such changes might be intuitively interpreted as a decrease in watershed-scale hydrologic system memory. Here, I investigate this hypothesis through a paired-catchment experiment. The serial correlation coefficient, a common metric of short-term time series memory, is applied to daily winter streamflow data from urbanizing and rural watersheds in the Puget Sound lowland of Washington State, USA. Statistical comparisons confirm that this metric shows highly significant decreases over time in the catchment undergoing land use change, but not in the control watershed, which remains rural over the hydrometric record. Moreover, the mean serial correlation coefficients are statistically indistinguishable between the two catchments over the early period of record, when both watersheds are largely rural, whereas the system memory is far weaker in the urbanized stream relative to the rural stream over the late period, following land use change in the former. The results appear readily interpretable in terms of the physical hydrologic changes typically associated with urbanization. The serial correlation coefficient thus appears to be an instructive measure of urbanization impacts for small streams in this region.

Hydrological aspects of the Mesoscale Alpine Programme: findings from field experiments and simulations

AbstractThe Mesoscale Alpine Programme (MAP) was a unique initiative to improve the understanding of processes involved in orographically influenced precipitation events and in the related emergence and mitigation of floods in the Alps. This study presents a summary of the results from investigations of hydrological relevance in areas to the south (Lago Maggiore watershed) and north (Ammer watershed) of the Alps. A special focus is given to the 1999 Special Observing Period (SOP) experiment. MAP has been an important dialogue platform between the hydrological and meteorological communities. A common goal has been the verification and the assessment of errors and sources of uncertainty of new coupled techniques for flood forecasting in mountainous environments. The ensemble multi‐model error in the hydrometeorological forecast, two days in advance, was 30% of the observed peaks and 15% of the observed runoff volumes, on average. To provide some insight on the relative importance and priority of observation strategies of hydrological factors such as soil moisture and reservoir volume in contributing to the flood intensity, the MAP project ‘Hydrology’ focused also on the role and magnitude of water storage in reservoirs. The presented analyses indicate that for the investigated basins and flood events, the storage reservoirs may retain, on average, up to 10% of the event runoff volume and are key elements contributing to the mitigation of peak runoff. The observed variability of soil moisture is large and therefore it has been confirmed that severe difficulties remain in instrumental monitoring of soil moisture in mountainous environments. The sensitivity of floods to soil moisture in the physio‐geographic context analysed here seems, however, to be minor, due to the shallow soil layer covering the underlying rocks and the relatively high permeability of soils which reduce the ‘hydrological memory’ of the soil cover in the mountain basins investigated. Copyright © 2007 Royal Meteorological Society

Interannual Variability of Deep-Layer Hydrologic Memory and Mechanisms of Its Influence on Surface Energy Fluxes

Abstract The characteristics of deep-layer terrestrial memory are explored using observed soil moisture data and simulated soil temperature from the Illinois Climate Network stations. Both soil moisture and soil temperature are characterized by exponential decay in amplitude, linear lag in phase, and increasing persistence with depth. Using spectral analysis, four dominant low-frequency modes are identified in the soil moisture variability. These signals have periods of about 12, 17, 34, and 60 months, which correspond to annual cycle, (4/3) ENSO, quasi-biennial (QB) ENSO, and quasi-quadrennial (QQ) ENSO signals, respectively. For deep layers, the interannual modes are dominant over the annual cycle, and vice versa for the near-surface layer. There are inherently two mechanisms by which deep-layer moisture impacts the surface fluxes. First, its temporal variability sets the lower boundary condition for the transfer of moisture and heat fluxes from the surface. Second, this temporal variability influences the uptake of moisture by plant roots, resulting in the variability of the transpiration and, therefore, the entire energy balance. Initial results suggest that this second mechanism may be more predominant.

Role of Terrestrial Hydrologic Memory in Modulating ENSO Impacts in North America

Abstract Relationships among the terrestrial hydrologic processes over the North American continent associated with the El Nino–Southern Oscillation (ENSO) are investigated using a large-area basin-scale land surface model driven by the European Centre for Medium-Range Weather Forecasts Re-Analyses 15-yr (1979–93) dataset. The modeling approach allows for the study of the relationships of ENSO with several hydrologic variables simultaneously, such as soil water storage, basin runoff, snow-water equivalent, and precipitation. The cross-correlation coefficients between terrestrial variables and the ENSO index are computed. The runoff from the northern part of North America was found to be most often negatively correlated with ENSO, and there are four distinct coherent regions over the continent where the runoff anomalies are positively correlated. The terrestrial systems have a delayed response to the ENSO signal, as compared to the precipitation, and the delay may range from a month to a season or longer. ...

Three-Dimensional Analytical Solutions for Unsaturated Seepage Problem

Transient, three-dimensional flux equations are solved by neglecting capillary forces in the first half of the paper, and in the second the steady-state seepage, where the capillary forces are significant, is treated. The following conclusions can be drawn by analysis of solutions for the cases of no capillary effects in unsaturated, anisotropic, layered slopes: (1) The seepage flow for these cases depends on the anisotropy of the soil. It further depends on the slope of the principal axis of the conductivity tensor. The seepage flow is completely independent of the slope of the soil surface. These findings make possible a new description of lateral flow mechanisms in slopes. Thus, the surprising result is that soil can possess a kind of “hydrological memory.” This is because the conductivity tensor can stay unchanged while the soil surface is altered. (2) Based on these findings, when a land fill cover is being designed, it is possible to obtain an optimal angle to minimize infiltration. Solutions for the three-dimensional seepage problem with axisymmetric boundary fluxes and significant capillary effects are obtained by the power-series method. An important group of solutions with noncontinuous (steplike) boundary fluxes is obtained by superposition of those solutions with constant boundary fluxes for semi-infinite and finite mediums.