Data and water balance estimation approaches for an agriculturally developing ungauged catchment: towards improved groundwater management

Author: Manh Hai Vu

Vu, Manh Hai, 2020 Data and water balance estimation approaches for an agriculturally developing ungauged catchment: towards improved groundwater management, Flinders University, College of Science and Engineering

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Groundwater is of fundamental importance as a fresh water resource for drinking water supply and irrigation. Groundwater is spatially and temporally variable and is in a dynamic (dis-)equilibrium with recharge, evapotranspiration, base-flow and abstractions for anthropogenic use. A number of factors, including natural variability and human activities, influence these flow components. Therefore, information and knowledge about these components is crucial for informing the sustainable use and development of groundwater resources, especially for agricultural catchments in developing countries. In many of these countries, groundwater demand is high, while technical and management expertise is lacking. Failure to appropriately manage groundwater resources based on data and scientifically tested methods often leads to groundwater depletion, pollution and ecological degradation of groundwater-dependent ecosystems.

This study aims to develop methods for understanding the groundwater budget in relation to its controlling factors in the context of a tropical agricultural headwater basin in the developing conditions of Vietnam. The methods developed for estimating the groundwater budget have to be commensurate with the ungauged conditions of this basin. Specifically, this study aims to (1) estimate the influence of the strong seasonal tropical climate and anthropogenic groundwater abstraction on the fluctuations of groundwater base-flow to streams; (2) develop a multi-method approach for estimation of groundwater abstraction for ungauged catchments; and (3) examine the response of the groundwater system to different scenarios of agricultural development, in which both crop patterns and groundwater demand and extraction vary.

The used methods and data consist of analysis tools, simulation models, literature (databases), as well as for this thesis field collected data. The data have been used to build a number of simulation models, more specifically a 2D cross-sectional unsaturated-saturated zone model, a spatially distributed monthly water balance model and a 3D saturated zone groundwater flow model.

The main results are:

1. 2D cross-sectional models have been developed for simulating the exchange fluxes between groundwater and surface water at three transects located upstream, midstream and downstream in the catchment. Variations of precipitation and anthropogenic groundwater abstraction were reflected by the changes in both groundwater and in-channel water levels as inputs for the models. Simulated fluxes varied both spatially and temporally: in addition to increasing from upstream to downstream, fluxes are high during the rainy season and decrease in the dry season. Groundwater discharges to streams most of the time, with exceptional losing conditions during intensive rainfall. The results indicate the strong influence of the seasonal pattern on precipitation for base-flow generation to the river, while groundwater abstractions have a smaller influence on the base-flow.

2. For an ungauged catchment, two approaches were developed for indirect quantification of groundwater abstraction based on ‘soft data’ from local knowledge and satellite-based land use data. In the first approach, the catchment’s average groundwater abstraction was estimated based on a qualitative field survey of groundwater level fluctuations, supplemented with information from the base-flow estimation of Chapter 2 and geographical and hydrogeological data for the catchment. In the second approach, distributed groundwater abstraction for the whole catchment was mapped based on land use data combined with local knowledge on cropping and irrigation practices, obtained by field surveys. The uncertainties associated with each approach were discussed and recommendations were made for low-cost management options to reduce possible uncertainties when estimating groundwater abstractions. The advantages and simplicity of these approaches make them attractive for application in other ungauged catchments.

3. For understanding the influences of land use change and climatic variability on catchment groundwater budgets, 12 potential land use scenarios were developed from the base scenario of current (2016) land use of the agriculturally dominant La Vi catchment. Three climatic conditions (i.e., dry, average and wet) were categorised from a 30-year time series of annual precipitation. A multi-model approach was used for testing 18 combinations of climate and land use conditions, comprising three climatic conditions and six land use scenarios (selected from those developed), including base scenarios. The WetSpass-M model was used for simulating spatial recharge and the MODFLOW 3D finite-difference groundwater flow model was employed for simulating a groundwater flow system, with recharge data taken from the WetSpass M output. Results from the models showed significant modifications, ranging from a 39% reduction to up to a 44% increase in the groundwater storage during the eight-month dry period for the selected land use scenarios. Seasonal climatic variation also causes a significant change in groundwater storage and base-flow to streams during the four month wet season. Specifically, groundwater storage varies between -44% and a 45%, while base-flow to streams varies between 71% and 192%. The results show that 15 of the 18 developed scenarios have to be classified as overexploited, as their groundwater abstraction ratio (i.e., proportion of abstraction to recharge) is higher than a sustainable ratio of groundwater abstraction of 0.35.

Keywords: Groundwater balance, agriculture, developing regions, ungauged catchment, estimating groundwater abstraction

Subject: Hydrology thesis

Thesis type: Doctor of Philosophy
Completed: 2020
School: College of Science and Engineering
Supervisor: Okke Batelaan