No abstract

Groundwater from unconfined chalk aquifers constitutes a major water resource in the UK. The unsaturated zone in such systems plays a crucial role in the hydrological cycle, determining the timing and magnitude of recharge, and the transport and fate of nutrients.However, despite more than three decades of study, our physical understanding of this system is incomplete. In this research, state of the art instrumentation provided high temporal resolution readings of soil moisture status, rainfall and actual evaporation from two sites in the Pang and Lambourn catchments (Berkshire, UK), for a continuous two year period (2004/5). A parsimonious, physically based model for the flow of water through the Chalk unsaturated zone, including a novel representation of the soil and weathered chalk layers, was developed. The parameters were identified by inverse modelling using field measurements of water content and matric potential. The model was driven by rainfall and evaporation data, and simulated fluxes throughout the profile (including the discrete matrix and fracture components), down to the water table (but not the water table response). Results showed that the model was able to reproduce closely the observed changes in soil moisture status. Recharge was predominantly through the matrix, and the recharge response was strongly attenuated with depth. However, the activation of fast recharge pathways through fractures in the Chalk unsaturated zone was highly sensitive to rainfall intensity. Relatively modest increases in rainfall led to dramatically different recharge patterns, with potentially important implications for groundwater flooding. The development and migration of zero flux planes with time and depth were simulated. The simulations also provided strong evidence that, for water table depths greater than 5 m, recharge fluxes persist throughout the entire year, even during drought conditions, with important implications for the calculation of specific yield from baseflow estimates and the representation of recharge in groundwater models.

Integrated hydro-economic models have been widely applied to water management problems in regions of intensive groundwater-fed irrigation. However, policy interpretations may be limited as most existing models do not explicitly consider two important aspects of observed irrigation decision making, namely the limits on instantaneous irrigation rates imposed by well yield and the intraseasonal structure of irrigation planning. We develop a new modeling approach for determining irrigation demand that is based on observed farmer behavior and captures the impacts on production and water use of both well yield and climate. Through a case study of irrigated corn production in the Texas High Plains region of the United States we predict optimal irrigation strategies under variable levels of groundwater supply, and assess the limits of existing models for predicting land and groundwater use decisions by farmers. Our results show that irrigation behavior exhibits complex nonlinear responses to changes in groundwater availability. Declining well yields induce large reductions in the optimal size of irrigated area and irrigation use as constraints on instantaneous application rates limit the ability to maintain sufficient soil moisture to avoid negative impacts on crop yield. We demonstrate that this important behavioral response to limited groundwater availability is not captured by existing modeling approaches, which therefore may be unreliable predictors of irrigation demand, agricultural profitability, and resilience to climate change and aquifer depletion.

[1] There is increasing acceptance of the importance of slow drainage from the unsaturated zone (SDUZ) when interpreting drawdown-time curves derived from pumping tests. Previous analytical solutions have generally assumed instantaneous drainage from the unsaturated zone. Such models typically underestimate the specific yield. Some authors have sought to account for SDUZ by assuming that drainage from the unsaturated zone declines exponentially with time, giving rise to an empirical delay index. These models tend to overestimate drawdown at early times and underestimate it during late times. More recently, the superposition of an arbitrary number of exponential models with different delay indices has been advocated, giving rise to an overparameterized and complicated empirical function. Following the work of Kroszynski and Dagan (1975), we obtain a new drainage function based on a linearized Richards' equation assuming that moisture content and hydraulic conductivity are exponential functions of pressure head. Furthermore, the drainage function can be incorporated into existing analytical solutions (e.g., Moench, 1997) with minor adjustment. The resulting model requires an additional three parameters: a moisture retention exponent, a hydraulic conductivity exponent, and the initial unsaturated zone thickness. The new drainage function can also be used in an empirical fashion with only one parameter (the other two are lost by assuming an infinitely deep unsaturated zone and that the moisture retention and relative permeability exponents are equal). Its applicability is demonstrated using pumping test data sets from Borden and Cape Cod. The results show improved consistency with the experimental data in comparison with previous studies.Citation: Mathias, S. A., and A. P. Butler (2006), Linearized Richards' equation approach to pumping test analysis in compressible aquifers, Water Resour. Res., 42, W06408,

5An exact solution for transient Forchheimer flow to a well does not cur-6 rently exist. However, this paper presents a set of approximate solutions, 7 which can be used as a framework for verifying future numerical models