Q1：After the arrival of hunter-gatherers in the southwestern region of North America, 

这半句话看不太懂它这个表达，半懂不懂的我在大致理解了这半句的句意后会这么写：After the hunter-gathers arrive in the southwestern of North America, 不清楚这两种表述在含义上或其他方面有何不同；而且，一旦两个介词短语连在一起，我压根判断不成都是谁修饰谁的，尤其是两个介词短语中后面那个，有时我看它会修饰前面紧挨其的介词短语中最后的那个N.修饰，有时又不是这样。

答：in the southwestern region of North America, 是介词短语作地点状语，北美洲的西南地区。

Q2：all involving different solutions to the Southwest’s fundamental problem:

首先，语法上，不是很理解这半句做什么成分，查了句子分析器里的解释，说这块是现在分词短语做句子状语，感觉不对啊；其次，句义上，所有农业都包括了关于西南根本问题的解决方案，这感觉翻译完不通顺啊~~

答：all 指代 several alternative types of agriculture. all 是several alternative types of agriculture的同位语，involving different solutions to the Southwest’s fundamental problem是现在分词短语作后置定语修饰all. 还有一种更容易的理解方式all involving different solutions to the Southwest’s fundamental problem=all which involve different solutions to the Southwest’s fundamental problem

Q3：to grow crops in an environment in which rainfall is so low and unpredictable that little or no farming is practiced there today.

首先，in which后面的这个从句，句子分析器里说是定从，但in which后面从句里的内容主干成分又不缺，我感觉是状从啊；其次，要是environment在从句里状语的话，它前面那个短语in an environment在主干中也做状语，这有点懵；最后，整个背景不是讲的是过去的事儿吗，从句里边的系动词怎么都用了一般现在时啊

答：这里的in which和关系副词where是一样的，an environment是先行词，这里的which指代的是an environment， 在这样一个环境中，rainfall is so low and unpredictable that little or no farming is practiced there today. 你说的没错，rainfall is so low and unpredictable that little or no farming is practiced there today.这句话是完整的，但是我们也加一个介词结构作地点状语，这里的in which 就是起到一个桥梁的作用，把前面的an environment和后面的rainfall is so low and unpredictable that little or no farming is practiced there today通过in which 联合在一起。不论是in which和where这句话都是定语从句，因为an environment需要在后面的句子中作成分！

用一般现在时，是因为这是一个大家公认的事实， in which rainfall is so low and unpredictable that little or no farming is practiced there today.