Natural recharge estimation and uncertainty analysis of an adjudicated groundwater basin using a regional-scale flow and subsidence model (Antelope Valley, California, USA)

Adam Siade,Peter Martin,Tracy Nishikawa

doi:10.1007/s10040-015-1281-y

Abstract

Groundwater has provided 50–90 % of the total water supply in Antelope Valley, California (USA). The associated groundwater-level declines have led the Los Angeles County Superior Court of California to recently rule that the Antelope Valley groundwater basin is in overdraft, i.e., annual pumpage exceeds annual recharge. Natural recharge consists primarily of mountain-front recharge and is an important component of the total groundwater budget in Antelope Valley. Therefore, natural recharge plays a major role in the Court’s decision. The exact quantity and distribution of natural recharge is uncertain, with total estimates from previous studies ranging from 37 to 200 gigaliters per year (GL/year). In order to better understand the uncertainty associated with natural recharge and to provide a tool for groundwater management, a numerical model of groundwater flow and land subsidence was developed. The transient model was calibrated using PEST with water-level and subsidence data; prior information was incorporated through the use of Tikhonov regularization. The calibrated estimate of natural recharge was 36 GL/year, which is appreciably less than the value used by the court (74 GL/year). The effect of parameter uncertainty on the estimation of natural recharge was addressed using the Null-Space Monte Carlo method. A Pareto trade-off method was also used to portray the reasonableness of larger natural recharge rates. The reasonableness of the 74 GL/year value and the effect of uncertain pumpage rates were also evaluated. The uncertainty analyses indicate that the total natural recharge likely ranges between 34.5 and 54.3 GL/year.

Highlights

Prior to 1972, groundwater had provided more than 90 % of the overall water supply in Antelope Valley, California, USA (Fig. 1)
A clear inflection point in the “overall” Pareto curve is observed when the natural recharge reaches about 54 gigaliters per year (GL/year). This value is consistent with the largest value observed during the null-space Monte Carlo (NSMC) analysis of about 49 GL/year, Fig. 19 Pareto curve or trade-off function that results from different rates of total average annual mountain-front recharge for observed early water levels, transient water levels, drawdown, and total subsidence in the Antelope Valley groundwater basin, California which suggests that the one-iteration approach for the recalibration step of the NSMC procedure may have been sufficient for adequately characterizing the predictive uncertainty associated with natural recharge for this study
A numerical groundwater-flow and land-subsidence model has been developed based on the model published by Leighton and Phillips (2003) to estimate the natural recharge and its associated uncertainty in the Antelope Valley, California

Summary

Introduction

Prior to 1972, groundwater had provided more than 90 % of the overall water supply in Antelope Valley, California, USA (Fig. 1). The three-layer model developed by Leighton and Phillips (2003) resulted in an estimate of total natural recharge of 37.4 GL/year and provided no quantitative estimate of the posterior uncertainty of this value. The quantity of natural recharge in Antelope Valley has been estimated in previous investigations based on rainfall, runoff, channel-geometry data, water-quality data, groundwater age dating, and groundwater-flow modeling. MODFLOW-NWT (Niswonger et al 2011), which is a Newton formulation of MODFLOW-2005 (Harbaugh 2005) in which an upstream weighted finite-difference method is employed resulting in greater stability when simulating complex nonlinear systems, especially systems containing model cells that transition from dry to wet or vice versa This is important for this study as most of the natural recharge occurs along the mountain boundaries where the basin fill is relatively thin and the model often contains only one active layer. The study conducted by CH2M HILL (2005) indicates that water levels in wells perforated

20 KILOMETERS

Findings

Conclusions