Why do the patients have Alkaline urine?

A 23- year old male has been admitted for vomiting over the last 2 days, with minimal fluid intake. Why do the patients have alkaline urine? He went to the emergency room of a nearby hospital and the following data was obtained upon examination and testing.
Hematology:
Hematocrit: 60%
plasma pH: 7.49 (normal is 7.35 to 7.45)
pCO2: 47 mmHg (normal is 35 to 45 mmHg)
HCO3 – : 31 (normal is 22 to 26 mEq/L)
Urinalysis Appearance: dark yellow to brown
Specific gravity: 1.025 (normal is 1.015 to 1.030)
Blood: negative
Protein: mild
pH 8.5