Here's why:

* Confined aquifers have a layer of impermeable material (like clay or rock) above and below the aquifer, preventing direct recharge from the surface. This layer acts like a "cap", keeping the water under pressure.

* Unconfined aquifers have a direct connection to the surface, allowing water to readily infiltrate and recharge the aquifer.

The Pecos Valley Aquifer has a layer of clay and silt above it, which acts as a confining layer, preventing direct surface recharge.