The answer to 'why is my drainage system brown patches' is that the brown patches are likely caused by a buildup of organic matter, mineral deposits, or microbial growth in the drainage system. This can happen over time as leaves, dirt, and other debris accumulate and decompose in the pipes or drainage areas.
The brown patches may indicate a blockage or slow drainage that is allowing water to pool and stagnate. This can lead to the growth of bacteria, algae, or other microorganisms that can discolor the drainage system. It's important to address the issue promptly to prevent further problems like mosquito breeding, unpleasant odors, or damage to the landscaping.