Detailed soil moisture outlook
Understanding the 'Detailed soil moisture outlook' page
The graph shows the water available for plant growth stored in soil at the weather station site at any time during the chosen period. Soil moisture is presented as an index with a value of 0% representing soil with no available water and a value of 100% for a saturated soil. The index is predicted from the simulation model GrowEst® based on actual rainfall and evapo-transpiration for a sandy loam soil. When soils at the site are not the sandy loam used in the simulations, the soil may have some available water when the value is 0% or may not be saturated when the value is 100%, but the index provides a scale to assess the relative soil water available for plant growth.
The green and black lines
The green line on the graph shows the weekly simulated soil moisture index from either 1 January of the current year or over the last 9 months, depending on the option chosen for "Soil moisture index". The index is calculated weekly from new climate records at the site. The right vertical grid line identifies the last simulated value, with the horizontal grid line indicating the current soil moisture index.
The green line can be compared with the black percentile lines to gauge how current soil moisture compares with all records for the site over the same months of the year. If the green line is close to the 50% line, soil moisture is close to the 'middle' range predicted using all climate observations made at the site over the same number of months. If the green line is on the 10% line, only 10% of all estimated soil moisture index values for the current week have been lower over the time records have been taken at the site. If the green line is on the 90% line, only 10% of all estimated soil moisture index values for the current week have been higher over the time records have been taken at the site.
The grey red and pink shading
The grey, red and pink shaded areas show the percentile lines for simulated soil moisture index over the next 3 months calculated from individual years recorded at the site, assuming the index values for all historical years are projected from the current value. These lines are 'smoothed' and updated monthly.
The edges of the grey and red (or pink) shading show the upper (90%) and lower (10%) range for soil moisture index for the next 3 months for individual years recorded at the site, assuming the valuesl for all historical years are projected from the current soil moisture index value. These projections are updated monthly.
The grey shading represents all years, whereas the red shading represents those years with the same sea surface temperature phase at the time of year as the last current rainfall measurements. Pink shading is used when the red and grey overlap. The red and pink shading may offer some insight into the soil moisture that could be expected over the next few months if rainfall is related to sea surface temperatures.
Historical years in the same sea surface temperature phase as for the last month of results are listed below the graph.
What to do with the information
The 'Detailed soil moisture outlook' page provides information that helps users to become more proactive in managing pasture feed supply and animal demand forward from today.
Assess your soil against the generic soil used in the model. (Only needs to be done once.)
- Soil moisture index is calculated for a soil with moderate drainage and organic matter content.
- High sand content and low organic matter content in your soil may mean that your soil moisture index is significantly lower than that projected for the site.
- High clay content and high organic matter may mean that your soil moisture index could be significantly higher than that projected for the site.
Assess the outlook for soil moisture by analysis of the red and grey areas on the graph.
- Assess the outlook for soil moisture by analysis of the red and grey areas on the graph.
- Adjust the outlook for soil moisture to reflect the observed difference between your property and the monitored site.
- Use the adjusted outlook to better understand the outlook for pasture growth. For example, high soil moisture coming out of spring into summer may increase expected growth from summer storms. Conversely low soil moisture reserves at the end of spring will reduce the impact of summer storms.
- Use the adjusted soil moisture index outlook to assess the opportunity for renovating pastures or sowing special purpose fodder crops.
- Look for opportunities to increase soil moisture over time by building plant litter to reduce rainfall run-off and evapo-transpiration, and to increase infiltration and soil water holding capacity through improved organic matter content and structure.
Glossary of terms
Percentile: The percentage of years when historical rainfall records (or the simulated soil moisture index or pasture growth index) fall below the percentile number. For example, for rainfall, the 50% line is the median or 'middle' observation from all weekly records - half the recorded values are lower and half the recorded values are higher for any specific week of the year. The 10 driest recorded values for any specific week in 100 years of records would fall below the 10% line and the 90 driest recorded values for the week would fall below the 90% line (the 10 wettest records would be above the 90% line).
The percentile curves are lines joining individual weekly percentile values. The accumulated rainfall curves do not represent actual years. For example, the line through the 10 percentile accumulated rainfall is not a 1 in 10 year, but the succession of 1 in 10 weeks.
Soil moisture index: Provides an indication of the likely wetness of the soil. The index is predicted from the simulation model GrowEst® based on recorded rainfall and other climatic information. A value of 0% indicates that the soil is completely dry whereas a value 100% indicates the soil is saturated.
Weeks: The day/month/year at the start of actual records is shown at the left of the x-axis. The following numbers represent day/month in subsequent months.
Similar Sea Surface Temperatures: Rainfall outlook is assessed from sea surface temperature phase (1-9) determined by the temperatures of the Pacific and Indian oceans. Historical years with the same sea surface temperature phase for the same month are shown as the red shaded area and listed at the bottom of the graph.
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