Calculating demand

[nahsan]

This is probably the most easiest question but I am having serious issue in calculating the demand in my model. I have some stock 'clothes in wardrobe'. Each year 50% goes to landfill and rest is re-used. While clothes stay in wardrobe for almost 7 years. On the basis of stocks I want vensim to calculate the new demand needed to produce every year. How can i model it in vensim. The way I am doing is using a delay function where my constant value of items needed (5 kg of clothes per year per capita) is minus with stock of last year. But I am getting demand for production in negative numbers. This can not be possible because demand of clothes can not go negative as people always buy clothes.
I hope someone can really help me out.

[tomfid]

It's hard to say what's going wrong without an example model.

However, one possibility is that what you're suggesting is not possible. If clothes last 7 years, then at most 1/7th can go to the landfill per year.

In any case, your decision rule for buying new clothes must:
- replace clothes discarded
- adjust the stock of clothes toward a goal

Often this implies something like:

clothes ordering = expected clothes discarding + (desired clothes - clothes)/time to adjust clothing stock
expected clothes discarding = SMOOTH( actual clothes discarding, time to perceive clothing discards )