GEOS4499 - Water in a Changing Climate Assignment

Assessment 3A - Hydrologic Data Analysis

Learning Objectives

This assessment will require students to understand how hydrologic monitoring can inform the conceptual models of ecosystems that underpin solutions to management challenges. 

At the completion of this assessment learners will have demonstrated their ability to:

• analyse and interpret measured hydrological data
• develop a conceptual hydrological model

• effectively communicate their understanding of a hydrological system through a written report

   

1.  Background

The degradation of freshwater systems by increasing salinity is a major challenge facing Western Australia. Salinisation of the Avon river, which enters the Upper Swan river at Walyunga National Park, arose due to poor land-management leading to secondary salinity in the inland wheatbelt catchments. Whilst secondary salinity is not an issue in the Swan Coastal Plain where our study site is, the salt loads from the Avon in addition to rising sea levels and decreasing rainfall trend have been driving changes in the seasonal movement of marine waters, with increasing penetration of the salt wedge further inland from the ocean (Huang et al. 2018).

Salinisation of river systems can cause shifts in floodplain vegetation which arise because of river water interaction with the floodplain. Depending on the salinity increase and the salt tolerance of the floodplain tree species, this can cause shifts in community structure. Eucalyptus rudis is an important species that has a natural range along the Swan river and its tributaries. This species has been reported to be in decline for over 15 years along the Swan river (Clay & Majer 2001), and more recently along one of the tributaries of the Swan river: the Guildford floodplains of the Helena river (Dundas & Mills 2011). It is currently unknown the exact role of water availability and salinity in the observed decline. An alternate hypothesis related to tree pathogens (e.g. Phytophthora) as a driver of decline is also under investigation.

E. rudis plays a vital role in the riparian ecosystem. Riparian woodland species are important filters that reduce nutrient transfers from the land into the river, which is a significant issue in the Swan river and its tributaries (excessive nutrients have historically resulted in noxious algal blooms). There is therefore a need to better understand what drives decline and if projected drying conditions for the region will make the pressures worse.

The City of Swan has identified our focus site on the Helena River near Guildford as an area of concern due to the decline of E. rudis. Part of the data set that you will work on was generated during a research investigation being undertaken with the WA Department of Biodiversity, Conservation and Attractions (DBCA) and UWA. The goal of this research was to understand the drivers of decline of E. rudis in relation to the changing hydrology regime at the Guildford floodplains along the Helena river.

Figure 1. Photo of dead or dying  E. Rudis at the study site

2. Helena River Study Site

Our study site is the riparian zone at the confluence of two rivers; the Swan River and the Helena River (Figure 1). Take some time to find this location on Google Earth or Google Maps so you understand where the study site is. The area is public open space immediately south of the township of Guildford; grassed “ovals” and vegetated areas that people use for recreation.

Figure 2. Google maps image showing study site on the southern side of Guidford at the confluence of the Swan and Helena Rivers

Even though we are quite a way in land, the Swan River at this location is still influenced by ocean tides - estuarine behavior (Figure 3). So the salinity varies as salt moves up the estuary from the coast. There is also water flowing towards the coast from inland rivers - predominantly the Avon River - which is generally fresher than the estuary. You can see the seasonal trend in Avon River flows by looking at the data you have been analysing in Exercise 1 and 2 from the Walyunga Gauge. 

Looking at a time-series of those data from the Walyunga gauge, can you see a shift to lower flows in 2001? 

Figure 3. Map of salinity monitoring points along the Swan River estuary (Thomson et al.  2001)

The balance of tidal and fluvial processes results in a seasonally varying distribution of salinity in the Swan River. The river is freshest during winter, as streamflow along the Avon River (and Ellen Brook) is highest, and most saline during summer when streamflows reduce to zero (Figure 4). 

Figure 4. Seasonal salinity profiles along the Swan River estuary (Thomson et al. 2001)

Looking in more detail at the riparian zone, the natural surface drainage of the site has been altered by artificial surface drainage channels (Figure 5). Water generally flows down hill; by altering the topography these engineering works have also impacted the movement of water across the site. The following map and LIDAR image shows the key features of the site, surface topography and locations of groundwater wells that were installed for this study (Figures 6 and 7).

Figure 5. An engineered drainage channel at the site

Figure 6. Map of elevations across the site and LIDAR data with locations of groundwater monitoring wells

Figure 7. Map of groundwater monitoring wells and key surface water features across the site

These groundwater monitoring wells are holes that have been drilled into the ground and lined with PVC pipes that have slots at the bottom to let the water in. All you can see from the land surface is the top of the PVC casing sticking up with a cap and sometimes a lock on it (Figure 8). 

Figure 8. The headworks of a groundwater monitoring well

Now watch this video recorded by Matt Hipsey as an introduction to the Helena River site.

3.   The task

During this assessment you will use measured field data to delineate the hydrologic pathways (surface and subsurface) that determine the distribution of water and salt in this floodplain system. You will use this information to develop a hydrological conceptual model of this site. You will prepare a report that describes the context of the investigation and summarises your findings and observations as suitable for a reader such as the City of Swan Environmental Management team.

On LMS you will find a number of data sets and tools to help you complete this task successfully.

• Time series data of water levels and salinity in the Swan River, Helena River and groundwater monitoring wells
• Rainfall data from the Perth airport BOM station
• Snapshots of soil water salinity across the site
• Aerial imagery to investigate hot-spots for declines in tree health

3.1 Develop our hypotheses

First, let’s develop some hypotheses about how salt might be moving through this system based on what we know so far. What pathways and processes do each of the numbered arrows in Figure 9 represent?

Figure 9. Schematic diagram of potential pathways for water and salt movement at the study site (unlabelled).

1. Streamwater inflow to the riparian zone - if this is bringing in salty water you would expect groundwater near the river to be saltiest and hydraulic gradient would have to be from the river to the water table for at least part of the year while the river was salty.
2. (And 3.) Plant root uptake of water from the capillary fringe and unsaturated zone - if this is important you would see higher salt concentrations in the soil than the groundwater
3. See 2.
4. Evapo-concentration of surface water and subsequent infiltration into the groundwater system - if this is important you would see salty groundwater on the down-gradient side of the seasonal paleochannel pond.
5. Flooding of salty river water and 6. subsequent infiltration into the subsurface - if this is important you would expect to see salinity highest near the surface and adjacent to the drainage channels that fill with flood water.

3.2 Time-series data for assessing hydraulic gradients 

The direction of water movement is determined by hydraulic gradients. The first thing you want to wrap your head around is what these gradients tell you about flow paths at the study site. Does water flow from the river into the subsurface, or is the river receiving groundwater discharge? To work this out, plot river water elevations (m AHD) and groundwater elevations (m AHD) on the same chart in Excel. 

• Which is higher - groundwater level or river stage? 
• Does the magnitude and direction hydraulic gradient change seasonally?

3.3 Salinity snapshots across the site

Surface water samples from across the site have previously been collected and analysed for electrical conductivity (EC), which is a measure of salinity. Let’s create a map to visualise the spatial variation in salinity across the study site. On LMS you will find instructions that take you through the steps to do this in QGIS, which can be downloaded here. If you are proficient in ArcGIS you can use that instead to create your map. From this mapping can you identify places in the landscape where salinity is accumulating?

You also have salinity in the rivers and groundwater bores - what information is there in these data? If you consider water levels and salinity in cross-section (Figure 10), how does this help you identify important processes? 

Figure 10. Cross-section of lithology across the study site

Remember that water can only move easily through permeable sediments. Vertical profiles of soil salinity (Figure 11) may help you identify top-down vs bottom-up processes.

Figure 11. Soil profiles and salinity measurements through the soil zone intersected during drilling for monitoring well installation.

3.4 Mapping vegetation change

There are now a range of spatial data products that have been generated from aerial imagery and remote sensing methods that we can use to help understand hydrological processes. One of these products is Nearmap, which allows us to access aerial imagery to investigate the changes in vegetation (and surface water distribution) over time (Figure 12).

Nearmap can be accessed at: http://maps.au.nearmap.com (login via UWA library Onesearch - see this webpage for details). Once you have successfully logged into Nearmap you can find the study site by searching for Guildford and then identifying the confluence of the Helena and Swan Rivers.

Figure 12. Nearmap interface showing study site in blue polygon.

Zoom in so that you can see the vegetation canopy clearly and then use the time bar at the top to look at changes over time (play button will scroll through all available images, clock icon will split the screen so you can compare two different dates.

• How has the vegetation cover changed over the years (compare similar times of year)?
• As you scroll through time does there seem to be a period of more rapid decline? Are there any periods of recovery?

You can also see the distribution of surface water on the Nearmap imagery - compare what you see with the following mapping of surface water inundation at the site (Figure 13).

Figure 13. Map showing area of inundation by water at seasonal high water levels (post-storm or flood). 

3.4 Conceptualizing SW-GW Interaction

Now that you have an understanding of the study site and the observation data, the next step is to develop your conceptual model of surface water - groundwater interaction. When trying to make sense of all this data from different locations and time periods some guiding questions can help you focus:

• How does salinity vary across the surface water sites? How does this relate to sources of water (Helena River, Stormwater from nearby suburbs, Swan estuary, Rain).
• How does water level and salinity vary in the groundwater based on distance from the river? Are levels in the aquifer higher or lower than the (mean) river level? What about salinity? 
• Which flow pathways drive the changes we see? Is water moving vertically or laterally? Consider how water levels in the piezometers vary relative to surface water. Think here Darcy’s Law and Hydraulic gradient.

4.   Submission Requirements

Your report will be assessed according to the marking rubric on LMS. Submissions not received by the due date will attract a late penalty as per UWA policy.

Your report should:

1.  Identify the pathways of water movement across the site (surface and subsurface) 
2. Articulate the connectivity between the surface water and groundwater
3. Assess the distribution of salinity in groundwater wells (piezometers) and surface water
4. Conceptualise the hydrologic processes influencing salt movement and accumulation across the site
5. Identify the locations of distressed and dead trees at the site
6. Articulate possible links between the hydrologic processes in your conceptual model and the declining health of E. Rudis

Your report should be written using clear, precise, concise scientific writing in English. Please do not submit text that has been translated from your first language if this is not English. Your report should have the following sections and be no more than 5000 words in total:

Introduction [1 page ]

• Background and context – why are we interested in this site?
• Map of site – where specifically are we looking at?
• Scope of the investigation – what is this report about?

Methods [2 pages]

• Data sources – external data/info we referred to
• Details of sampling and measurement infrastructure (locations, screen intervals, what was measured, for how long and at what temporal resolution.
• Data analysis approach, as relevant – any processing or “higher” analysis we did to the data

Results and findings [5 pages max ]

• Describe results quantitatively - be precise, use numbers.
• Map of surface water salinity
• Time-series graphs of water levels, flows and salinities (or EC)

• Don’t repeat statements that relate to study objectives or methods

   

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Discussion [1 page] 

• Conceptual model of surface and groundwater flow and salt pathways; consider all information in this, including past data and reports and nearmap imagery. Consider how summer may be different to winter.
• Interpretation of the above data in light of the observed tree-decline locations
• Suggestions for improved monitoring and further investigations; dot-points OK here.

Conclusion [1/2 page]

• A brief conclusion that links back to the objective(s) you stated in your introduction