Evaluation of the impacts of deep open drains on groundwater levels in the wheatbelt of Western Australia

Abstract

Abstract. Over one million hectares of the wheatbelt of Western Australia (WA) are affected by secondary salinisation and this area is expected to increase to between 3 and 5 million hectares if current trends continue. Deep open drains, as an engineering solution to dryland salinity, have been promoted over the past few decades; however, the results of initial experiments were variable and no thorough analysis has been done. This research quantifies the effects of deep open drains on shallow and deep groundwater at farm and subcatchment level. Analysis of rainfall data showed that the only dry year (below average rainfall) after the construction of drainage in the Narembeen area of WA (in 1998 and 1999) was 2002. The dry year caused some decline in groundwater levels in the undrained areas but had no significant impact in the drained areas. The study found that the effect of drains on the groundwater levels was particularly significant if the initial water levels were well above the drain bed level, permeable materials were encountered, and drain depth was adequate (2.0–3.0 m). Visual observations and evidence derived from this study area suggested that if the drain depth cut through more permeable, macropore-dominated siliceous and ferruginous hardpans, which exist 1.5–3 m from the soil surface, its efficiency exceeded that predicted by simple drainage theory based on bulk soil texture. The effect of drains often extended to distances away (>200 m) from the drain. Immediately following construction, drains had a high discharge rate until a new hydrologic equilibrium was reached. After equilibrium, flow largely comprised regional groundwater discharge and was supplemented by quick responses driven by rainfall recharge. Comparison between the hydrology of the drained and undrained areas in the Wakeman subcatchment showed that, in the valley floors of the drained areas, the water levels fluctuated mainly between 1.5 and 2.5 m of the soil surface during most of the year. In the valley floors of the undrained areas, they fluctuated between 0 and 1 m of the soil surface. The impact of an extreme rainfall event (or unusual wet season) on drain performance was predicted to vary with distance from the drain. Within 100 m from the drain, water levels declined relatively quickly, whereas it took a year before the water levels at 200–300 m away from the drain responded. The main guidelines that can be recommended based on the results from this study are the drain depth and importance of ferricrete layer. In order to be effective, a drain should be more than 2 m deep and it should cut through the ferricrete layer that exists in many landscapes in the wheatbelt.

Impacts of deep open drains on water quality and biodiversity of receiving waterways in the Wheatbelt of Western Australia

“…Of these, deep drains (2-3 m) are seen by farmers as the most likely solution to succeed, and it has been estimated that over 12,000 km of drains and banks have been installed (Dogramaci & Degens, 2003;Ali et al, 2004). Almost all of these drains discharge into streams and rivers and little consideration has previously been given to the ecological impacts of these discharges on already altered aquatic ecosystems.…”

“…As is the case for many deep drains in the Wheatbelt, these have been constructed to alleviate the effects of salinity on both township infrastructure and agricultural productivity. However, many of these drains have been constructed with limited planning and design, and with little or no understanding of their effectiveness or downstream impacts (Ali et al, 2004). In the case of the Narembeen drains, their effectiveness in lowering groundwater levels has been assessed by scientists (Ali et al, 2004), but the downstream effects of these drains has not received attention.…”

“…In the case of the Narembeen drains, their effectiveness in lowering groundwater levels has been assessed by scientists (Ali et al, 2004), but the downstream effects of these drains has not received attention. Since the discharge from these drains was found to be acidic (pH ranging from 2 to 4), highly saline (electrical conductivity values ranging from 4,000 to 10,000 mS/m), and carrying significant levels of heavy metals such as iron and aluminium (about 110-120 mg/l at some sites) (Ali et al, 2004), the lack of information on downstream impacts is of concern.…”

“…As part of this process, the downstream impacts of drains and pumping have been assessed in four catchments, including in the Wakeman Creek subcatchment near the Wheatbelt town of Narembeen. Deep drains have been constructed at various times during the period 1998-2001 in the Wakeman subcatchment near Narembeen (Ali et al, 2004), resulting in an extensive local deep drain network of approximately 100 km in length. During 2005During -2006, daily flow rate from this drainage complex usually ranged between 0 and 15 ML/day, but 'unseasonal' rainfall in summer 2006 resulted in a high of 25 ML/day being recorded during this period.…”

Remote sensing for assessing the zone of benefit where deep drains improve productivity of land affected by shallow saline groundwater

“…The area of benefit is a measure of the drain's efficiency. It is generally expressed as the lateral extent of the drain's influence on the water table and vegetation and is dependent on soil and regolith parameters such as hydraulic conductivity (Ali et al, 2004). Previous studies at a deep drainage site at Narembeen in the Western Australian wheatbelt, reported that one year after the drain was installed, groundwater levels dropped to below 1.5 m for a distance of 200-300 m from the drain and root zone salinity decreased for a distance up to 100 m from the drain (Ali et al 2004).…”