DROUGHT AND DOMESTIC WATER SUPPLY

THE PROBLEM CONTEXT: A CASE STUDY AS AN EXAMPLE

Residential expansion into rural areas adjacent to Rotorua is becoming increasingly constrained by water supply.  With the rising demands for water, the pressure on existing water supplies in the region is increasing and reaches crisis proportions during prolonged drought periods.  To make matters worse, regional projections of future climate change suggest a drying trend for the Bay of Plenty.


For these reasons, a new subdivision in the outskirts of Rotorua is seeking to become self-sufficient in water supply. This will be accomplished principally through the installation of individual home water tanks which are fed from roof runoff.  

The size and configuration of the water tank systems have to be decided before development approval is given for the subdivision. The decision will depend on such factors as climate variability and change, anticipated willingness of homeowners to modify their water demands, cost, and the degree of risk that the homeowners are willing to accept.


Your job is to conduct an initial analysis of such factors and advise council accordingly. If you are too stringent in your recommendations, the subdivision developers will scream that the costs are too high. If you skimp, you will surely draw the wrath of the public when the tanks run dry…      


Your job is divided into two parts:  


Part 1 involves examining the possible future changes in regional rainfall during the low rainfall season, i.e. Jan-Mar.


Part 2 involves an analysis for recommending the design of water tank systems, taking account of current rainfall variability and future change.


For both parts, your planning horizon is 50 years (i.e. to the year 2065)


PART 1:

SPATIAL PATTERNS OF RAINFALL CHANGE


TASKS:


(1)  Examine the regional variation in rainfall under current climate (for your particular area):


Steps for Using SimCLIM 4.0 for Desktop:  


Make certain that you have first selected the Bay of Plenty from the drop-down menu at the far right-side of the screen. Then choose


the Spatial Scenario Generator option





and select Precipitation, then the Linked tab; choose the year “1995” as the baseline year, which gives 30-year historical averages;  and select the months Jan, Feb, Mar inclusive, then Generate.




You might have to wait a bit for the image to appear. By dragging the borders of the image you can enlarge the image for the Rotorua area.




The coordinates of your new subdivision are: lat 2838604 and 6304643.  As you move the cursor over the image, its coordinates are registered as the first two numbers in the top-right portion of the screen.  The last number is rainfall amount (in mm).  



What is the average dry season (Jan-Mar) rainfall for your subdivision location?

Approximately 293mm of rainfall in the dry season.


(2)  Construct a scenario of climate change for precipitation:


When it comes to water, government agencies are well aware that the public prefers wide safety margins for dependable supply.  Thus, from the range of uncertainty, you are advised to be cautious and develop a “conservative” scenario of possible future changes in rainfall.  Better to be on the safe side…

Return to the Spatial Scenario Generator and select Precipitation and select:


Year:  2065

GCM:  Ensemble

Global Projection:  RCP8.5 (a high projection of greenhouse gas concentrations)

Climate Sensitivity: HIGH (at the high end of modeling uncertainties)

Select months Jan, Feb and Mar inclusive, then Generate.


Enlarge the image for the study area. Position the cursor over the location of the proposed subdivision.


Bay of Plenty 2065 Precipitation (Jan-Mar scenario)



Rotorua study area 2065 Precipitation (Jan-Mar scenario)



What is the projected percentage change in rainfall for this location?


Map of the projected rainfall change between baseline and 2065 scenarios. This shows a projected decrease in rainfall of approximately 6-8% by 2065.


Produced by generating 1995 (image 1) and 2065 (image 2) scenarios, and subtracting image 2 from image 1 (using the calculator function), then dividing by image 1 and multiplying the result by 100.



Change in precipitation by 2065, produced by selecting ‘Output change from baseline’ in the scenario generator tool. (However, change is negative, though values here are positive…)


What are the general implications for water supply and demand?

The projections are for a general decrease in precipitation over the dry season that would be expected to result in an overall decrease in supply and increase in demand (linked to such uses residential sprinkler systems and other domestic uses such as bathing. (Large scale irrigation systems will also demand more water to compensate for lower water availability in the soil, and increased demand by hydropower stations to generate more power, for increase cooling systems in the region).



PART 2:

ANALYSIS OF WATER TANK SYSTEMS



This part of the analysis involves the use of a Water Tank model, which is attached to, and driven by, the climate scenario generator of SimCLIM.  This model simulates the performance of a water tank system using time-series rainfall data. Please use the Water Tank model to analyse the adequacy of design features of the water tank systems for the subdivision.


TASKS:


(1) Analyse the adequacy of the subdivision developer’s plans under current climate


Steps for Using SimCLIM:  


On the Main Menu, choose NZ Bay of Plenty (far right-hand side of screen). From the Tools drop-down menu, select the Run an impact model option, then select the, Rain water collection model from the Show All tab.


On the screen, you should now have a dialogue box and a map (grab the lower border and drag down to get the full view). You are first offered Station options. Of the choices offered therein, choose:


  • Station:  Rotorua (this is the closest station to the subdivision and has a similar rainfall regime)
  • Start and End Years:  1964 and 1990 (this gives a 26-year record without missing data and is  very close to the 30-year period recommended by World Meteorological Organisation for climatological analyses)


Now switch to the other tab called Model Inputs.  Here is where you set the model parameter values. Figure 1 (see last page) provides a brief explanation of the parameters, the developer’s assumptions, and some alternative options and their costs.  The values consistent with the subdivision plans are:


Daily water consumption (litres):  1000

Water tank size (litres):  66,000

Water catchment area (m2):  210

Initial water storage (%):  20

Tolerance threshold for empty tank in day(s):  2


Select Run Model. The model has now used 26 years of daily rainfall data to simulate the system’s performance.


In the output box, select the Results tab. There are two outputs, as explained in Figure 1.  These have to be interpreted subjectively.  In your judgment:



As shown in this simulation, is the risk of an extreme prolonged period without tank water over a 26-year period acceptable?

The longest period of an empty tank between the years 1964-1990 is 66 days. This is unacceptable.


Is the number of occurrences of system failure within a 27-year period acceptable?

994 times in a 100-year period of the system failure exceeding the tolerance threshold. This would mean approximately 258 failures in the 26-year period, which would seem unacceptable.


(2) Re-design the system to meet your requirements for an acceptable level of risk.


As suggested in Figure 1, there are opportunities for modifying supply and demand – at a cost, however.  You should not wish to place undue financial burdens on the developer, but, on the other hand, you have a responsibility to consider the risks to the public. There will always be a residual risk of running out of water. The issue is, what is acceptable?


Using the guidance provided in Figure 1, systematically alter the parameters of the system until you achieve outputs that you consider acceptable. Make note of the parameter values and any extra costs that are entailed in modifying the system.



Adaptation measure

Extra Costs ($)


Daily water consumption (L)

Water tank size (L)

Water catchment area (m2)

Initial water storage %

Longest period of an empty tank = 1 day


Freq of empty tanks in every 100-year period = 0

500

66,000

250

20

2000

Longest period of an empty tank = 0


Freq of empty tanks in every 100-year period

200

66,000

210

20

Cost of implementing the additional water conservation technologies

Longest period of an empty tank = 0


Freq of empty tanks in every 100-year period = 0

500

66,000

210

50

Cost of implementing the additional water conservation technologies

Longest period of an empty tank = 0


Freq of empty tanks in every 100-year period = 0

180

210

66,000

50

1500



(3) Examine the implications of climate change

From Part I of this exercise, you constructed a scenario of the percentage change in rainfall for the subdivision for the year 2057.  After ensuring that all other parameter values are consistent with your re-design of the water system, enter this percentage change in rainfall into the box Rainfall Change in Percentage (do not forget to put the negative sign before the number)


Rainfall change percentage for the subdivision area = -8.8%


How does climate change affect the outputs of the simulation. What are the implications for your acceptable level of risk?


Climate change affects the outputs such that the:

* Longest period of an empty tank in days = 74, and

* Frequency of empty tanks in every 100 years (exceeding tolerance threshold) = 1253

This presents a completely unacceptable level of risk to the community into the future and would require significant adaptation options to be implemented to mitigate the effects to an acceptable level.



(4) Suggest further modifications to the design in order to adapt to climate change


In order to maintain your acceptable risk level (as determined in Task 2 above) under climate change, it will be necessary to make further modifications of the systems -- in other words, adaptation.


Fine-tune the parameters in order to reduce the risks to acceptable levels. Make note of the incremental costs involved.






What are the adaptation measures you chose and their incremental costs?



Adaptation measure

Extra Costs ($)


Daily water consumption (L)

Water tank size (L)

Water catchment area (m2)

Initial water storage %

Longest period of an empty tank = 0 day


Freq of empty tanks in every 100-year period = 0

500

66,000

210

50

Some water conservation technologies implemented

Longest period of an empty tank = 0


Freq of empty tanks in every 100-year period = 0

500

66,000

250

50

Cost of implementing the additional water conservation technologies

+ 2000 for extra 40m2

Longest period of an empty tank = 0


Freq of empty tanks in every 100-year period = 0

200

66,000

250

50

Cost of implementing the additional water conservation technologies

+ 2000 for extra 40m2

Longest period of an empty tank = 0


Freq of empty tanks in every 100-year period = 0

180

66,000

210

50

1500 for additional water conservation technologies and drip-feed irrigation



Given the uncertainties of surrounding the climate change scenarios, do you have any suggestions regarding the current commitment to, and timing of, the adaptation measures?


The only measure that seems to significantly reduce the risk of empty tanks and failures, is to reduce water consumption. This is a no regret option, and the suggestion is that these measures need to be put in place so as to reduce current vulnerability to empty tanks and systems failures. Reducing water consumption to 500L, already reduces the number of days of an empty tank to 0, and the frequency of empty tanks in every 100 years (exceeding the tolerance threshold) to 0 as well. This could be done in the next few years and you will already have ‘ensured’ that you maintain a 0 ‘empty tanks’ situation with a –potential -8.8% reduction in precipitation by 2065. Whether it eventuates on not, you are already more resilient under the current conditions, and so it is wise to implement this strategy also from the point of view of more sustainable resource consumption.




Figure 1:  The water tank model inputs and outputs.