Soil Moisture Budget

From H2Know-how Manual

AWT have developed the H2KnOw-how Soil Moisture Budget to assist with operating and monitoring the Waihou wastewater land irrigation scheme. This is a comprehensive tool for assisting operational personnel in understanding and interpreting soil moisture information to determine whether irrigation is appropriate and to ensure that the irrigation scheme meets resource consent conditions.

Soil Moisture Budget Start Guide

The following sections will walk users through working with the soil moisture budget module in H2knOw-how.

Logging On

To view the H2KnOw-how Soil Moisture Budget, Login to H2KnOw-how with your provided credentials. Then select ‘Process Design’. H2KnOw-how displays the irrigation scheme (and treatment plant) on the Map Page, where the irrigation zones and plant features are shown in their respective locations.

Figure 1: Screenshot of Map Page showing irrigation zones and plant features.

The Map Page is your portal to all the information stored in the Soil Moisture Budget. To find out more about the features on the Map Page, click here.

Navigating Plant Features

A Plant Feature is any item on the Map Page that contains data (eg. irrigation zone, moisture probe, weather station etc.). Clicking on a plant feature will open up a pop-up window with links to information relating to that plant feature.

There are five plant features available:

  1. Wastewater Sample - which contains real time soil moisture probe data.
  2. Pond - no information currently exists for this feature, but pond level information could be incorporated into the model if necessary in future.
  3. Weather Station - contains data on actual and potential evapotranspiration, actual and forecasted wind speed, rainfall and wind direction.
  4. Flow Meter - there are two flow meters:
    1. Plant outflow which contains plant irrigation outflow and the 7 day rolling total of irrigation outflow data,
    2. Stream monitor which contains stream flow and level data.
  5. Irrigation Zones - these contain data on the irrigation area, field capacity used, and real time data on soil moisture, soil storage used and soil storage available.

An example of the irrigation zone feature is provided in the following screenshot.

Figure 2:  Plant Feature pop-up window.
  1. Data Table – Use this to view data in table format, view statistics and set / adjust threshold values
  2. Graph – Use this to view data graphically and add traces
  3. Edit – Use this to edit the Plant Feature name and description
  4. Properties – Use this to edit irrigation area or field capacity
  5. Notes and Documents – Add notes and documents related to the plant feature here
  6. Variables – These are the variables that are stored in this plant feature

Using the Soil Moisture Budget

The following sections describe how the soil moisture budget works, where the data comes from, and how to interpret the budget data.

What is a Soil Moisture Budget?

A soil moisture budget estimates the amount of moisture that is in the soil based on soil moisture readings, rainfall and evpotranspiration. The method for calculating the soil moisture budget is:

Yesterday's soil moisture - yesterday's evapotranspiration + yesterday's rainfall + yesterday's irrigated wastewater = today's total soil moisture level.

The estimated soil moisture is compared against the soil field capacity, which is amount of soil moisture or water content held in the soil after excess water has drained away and the rate of downward movement has decreased. If the moisture budget is below field capacity, irrigation can occur. The budget will calculate how much volume you can irrigate onto each irrigation zone. If the budget exceeds field capacity, irrigation to land should be avoided as soil saturation will occur and increased risk of ponding, runoff and leaching will arise.

A soil survey undertaken in December 2009 (Orbell, 2009) estimated that the soil field capacity at the Waihou irrigation site was approximately 450mm (or 45% soil moisture content). To add a factor of safety, we have set the threshold soil field capacity at 400mm (40%). It is recommended that a more accurate soil assessment be undertaken to determine the field capacity in each zone.

How much can I irrigate?

On the Map Page, the irrigation zones will beare colour coded to indicate if you can irrigateirrigation can occur and what volume. This is determined from the soil moisture probe information, the 7 day rolling average discharge volume, rainfall and evapotranspiration data:

  • Blue means irrigation can occur up to the volume displayed on the map (kL is the same as m3).
  • Orange means there may be volume available, however operator discretion should be used as the amount of irrigation may be limited by the weekly discharge volume limit.
  • Red means irrigation to land should not occur because the soil moisture budget is exceeding field capacity and there is a risk of ponding or runoff occurring.
Figure 3:Example irrigation zones colour coded to indicate irrigation possibility and volumes.

Other key features to be aware of when deciding whether to irrigate is forecasted wind speed. This should also be checked on a daily basis to ensure irrigation does not occur when wind speeds are greater than 12m/s.

Viewing Graphs

In the Plant Features menu, click on the graph icon to view data in graphical format.

Figure 4:Click to view graphs.

The following sections describe the variables contained within each plant feature which can be viewed as graphs. From one graph page, you can view graphs of multiple variables by selecting the 'Add a Trace' icon and then selecting the sampling point and variable you want to add.

Irrigation Zones (Soil Moisture and Available Irrigation Volume)

The Irrigation Zone graph page contains the following information:

  • Soil Moisture (%) - Average daily soil moisture probe reading (average of the 200mm and 300mm probe readings).
  • Soil Storage Used (%) - Daily soil moisture budget. This line should stay between the wilting point and field capacity thresholds (15% and 40% respectively). If this line exceeds 40%, no irrigation can occur.
  • Soil Storage Available (kL) - The volume that is available in the soil for irrigation. The lower the soil moisture, the greater the volume that can be applied.

This page allows you to view the historic soil moisture and available irrigation volume trend.

Example: The screenshot below shows that irrigation could not occur on 21st April because the soil moisture budget was too high.

Figure 5:Soil moisture budget and irrigation volume graph.

Moisture Probe

The 'Moisture Probe' graphs contain the raw data from the soil moisture probes (at 200mm and 300mm depths) as well as an average soil moisture (average of the two soil moisture readings). The moisture probe readings are updated on HH2KnOw-how every 10 minutes to enable real-time monitoring. The average soil moisture is also shown in the Irrigation Zone graph page.

Fig15 SoilMoistureProbe.jpg

Weather Station

The ‘Base Station’ graph contains historical weather information, including:

  • Previous day’s rainfall - uploaded from Waihou WWTP weather station daily at 11:59PM.
  • Previous day's potential evapotranspiration (PET) - uploaded from NIWA daily at 9:00AM.
  • Previous day's actual evapotranspiration - estimated from PET multiplied by a coefficient of evaporation of 0.7.
  • Today's actual wind speed - uploaded from Waihou WWTP weather station approximately every 5 minutes.
  • Today’s predicted wind speed - uploaded from Te Aroha weather station via every 3 hours.

The H2KnOw-how calculator uses the previous day’s data to calculate today’s moisture budget.

The resource consent specifies that effluent discharge to land should not occur during high winds and a wind speed limit of 12m/s applies. A threshold has been placed on the Base Station wind speed graph. If discharge to land is not possible, the Operator should then check the Stream Monitor graph to ensure that stream flow is high enough to allow discharge to water (see 3.3.5 below).

Flow Meter - Plant Outflow

The ‘Plant Outflow’ graph contains the following information:

  • Previous day's plant irrigation outflow m3/d - measured from the irrigation pump station wet well and uploaded daily at 11:59PM.
  • Plant outflow m3/hr – total plant outflow measured from UV outlet and uploaded approximately every 5 minutes.
  • Plant outflow (gravel) m3/hr – total plant outflow measured at outlet of gravel beds prior to discharging to the stream and uploaded approximately every 5 minutes.
  • Plant irrigation outflow (7 day rolling total) m3 – calculated by H2KnOw-how. See here for more information.

The resource consent specifies a weekly discharge volume of 350m3 that can be discharged to land. A threshold has been placed on the Plant irrigation outflow (7 day rolling total). If discharge to land is not possible due to potential exceedence of this 7 day rolling total, then the Operator should check the Stream Monitor graph to ensure that stream flow is high enough to allow discharge to water (see 4.1 below).

Flow Meter - Stream Monitor

The ‘Stream Monitor’ contains the following information:

  • Stream Level m – measured from the stream gauge positioned on the land irrigation bridge. Measurements are taken and uploaded every 15 minutes.
  • Stream flow L/s – calculated by H2KnOw-how based on stage-rating discharge curve (AWT Water, Waiwhero Stream Gauging memo, October 2013).

The resource consent application stated that the mean annual 7-day low flow was approximately 25L/s; this flow was used to assess effects on the Waiwhero Stream from the WWTP discharge. It is recommended that discharge to the stream does not occur during low flow conditions. A threshold has been placed on the graph page. If discharge to land is not possible, the Operator should then check the Stream Monitor graph to ensure that stream flow is high enough to allow discharge to water.

Figure 6:Stream flow graph.

Example: The screenshot below shows stream flow over the period shown was well above low flow 25l/s, and thus if land irrigation could not occur, then discharge to the stream would be permitted.

Viewing the Data Table

In the Plant Features menu, click on the table icon to view the raw data, statistics and other data viewing tools

Figure 7:Click to view data table.

This will open up the Data Table view.

Example: The screenshot below shows the irrigation zone C soil moisture statistics for the period between 21 April and 21 May.

Figure 8:Data table overview.
  1. Set threshold values by clicking ‘Add New’ or edit existing values by changing the ‘Value’ field. Click Save to save edits. The Field Capacity and Plant Available Water thresholds have been based on the soil survey undertaken in December 2009 (Orbell, 2009) for the purpose of the Waihou WWTP discharge consent application (MPDC, 2009).
  2. Filter to view data from specific periods by selecting the from and to dates.
  3. Statistics are calculated in H2KnOw-how for the selected period.
  4. Raw data from H2KnOw-how is shown here in tabular format.
  5. Download data by selecting the date range and type of data (raw, average, min, max etc).
  6. You can navigate to another plant feature by selecting it from the drop down menu.

Waihou WWTP Consent Compliance Monitoring

Although touched on earlier, the following presents particular consenting requirements that must be met as part of the land application scheme.

Seven Day Rolling Total Irrigation Volume

The Waihou WWTP discharge consent specifies a maximum volume of effluent that can be discharged to land of 350m3/week. The Soil Moisture Budget automatically calculates and graphs the 7 day rolling total irrigation volume. Access this graph by clicking on ‘Plant Outflow’ flow meter and then ‘Plant Irrigation Outflow (7 day rolling total)’. Please note that the unit kL is the same as m3.

This graph shows the total volume irrigated to land in the previous 7 days. A threshold has been set at 350m3.

Figure 9:Seven day rolling total flow graph.
  • If the line exceeds this threshold, then there has been a breach of the consent. The example above shows between Dec 2013 and April 2014, there were two exceedances of the 7 day rolling total flow limit.
  • The zone will be colour coded orange and will show 0kL of available irrigation volume – irrigation should not occur.
  • If irrigating a certain zone today is likely to cause a breach of this consent condition, the zone will be colour coded orange – operator’s discretion should be used when selecting whether to irrigate and which zone to irrigate to.

Wind Speed Monitoring

The consent specifies that irrigation should not occur if the wind speed exceeds 12m/s, as high winds could result in spray drift into adjacent properties. To check the forecasted wind speed for the day, click on ‘Weather Station’ and then ‘Wind Speed (Daily Forecast)’.

This graph shows the forecasted wind speed for the next five days. A threshold has been set at 12m/s. Actual wind speed can also be graphed by checking the ‘Plot’ box.

Figure 10:Wind speed graph.
  • If the wind speed is forecasted to exceed 12m/s, no irrigation should occur on that day. Operator’s discretion should be used.

Example: The example above shows that forecasted wind speed is not expected increase above 12m/s so therefore wind speeds is not a factor preventing irrigation to occur over this period.

Adding Notes and Documents

The H2KnOw-how database can be used to store notes and documents related to the site. For example, installation reports, calibration reports and consent monitoring reports.

  1. To add notes or documents to a specific feature / zone, click on the feature:
    • To add a note, click the Notes Tab and select ‘Add a Note’. Write the note and select if it is public (viewable by anyone with access to this site) or private (viewable to the person entering the note only).
      Figure 11:Add a note.

    • To add a document, click the Documents Tab and select ‘Add a Document’. Fill in the fields and select a file. Press Save to upload the document.
      Figure 12:Add a document.

  2. To add documents to the project page (not specific to a plant feature):
    • Click on the Home icon
    • Click 'Files and Documents' > ‘Waihou WTP’
    • Click ‘Add Document’, fill in the fields and select a file. Press save to upload the document.